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Petrogenesis of typical Mesozoic and Cenozoic volcanic rocks from the North China Craton: New evidence from Hf isotopic studies
Abstract
High precision Hf isotopic data are firstly reported for the typical Mesozoic and Cenozoic volcanic rocks (alkaline basalts, tholeiites, and high-Mg andesites) from the North China Craton in this paper. The results show that these Mesozoic and Cenozoic volcanic rocks have completely different Hf isotopic compositions with a large range in ε Hf (t) values (-18.4 ∼ 9.6) with an excellently positive correlation between Hf and Nd isotopes. Among them, the Cenozoic Changle-Linqu and Late Cretaceous Jianguo (∼ 105 Ma) alkaline basalts possess very high Hf and Nd isotopic compositions (ε Hf (t) = 6.8 ∼ 9.6; ε Nd (t) = 0.4 ∼ 4.9), indicating their derivation from the undifferentiated asthenospheric source. Early Cretaceous Fangcheng (125 Ma) alkaline basalts and tholeiites have extremely low Hf and Nd isotopic ratios (ε Hf (t) = -15.6 ∼ -18.4; ε Nd(t) = - 12.8 ∼ -14.4), representing their origination from an enriched sub-continental lithospheric mantle. Wulahada (142 Ma) high-Mg andesites show relatively lower Hf and Nd isotopic ratios (ε Hf(t) = -2.1 ∼ -2.6), implying a derivation from old refractory lithospheric mantle contaminated with melt fulid released from subducted crustal materials. Relatively larger variations in Hf isotopic compositions of basalts (ε Hf (t)/ε Nd (t) > 1) demonstrate that the Hf isotopes could be better tracer of mantle sources than the Nd isotopes.
... Basalts of Mesozoic age are widespread in eastern China and are interpreted as resulting from the lithospheric thinning Zhang et al., , 2004Yang et al., 2006;Gao et al., 2008;Y.S. Liu et al., 2008). Cenozoic basalts of varying age are also abundant in this region. ...
... Hf and Nd isotopes are different between lithospheric mantle and asthenospheric mantle (Griffin et al., 2000), which are powerful in tracing the source of basalts. However, because of D Lu /D Hf N D Sm /D Nd , Lu-Hf isotopic system is more sensitive to mineralogy in the source and melting residues during partial melting (Johnson et al., 1996;Vervoort and Patchett, 1996;Schmitz et al., 2004), implying that a significant fractionation of Lu/Hf ratio can occur, which therefore can generate a broad range of Hf isotopic variation (e.g., Yang et al., 2006) and could better trace the source and source histories. In addition, the nature of the source of Cenozoic basalts in the NCC is controversial. ...
... The N110 Ma basalts are isotopically enriched with ε Nd b 0 and ε Hf b 0. They all fall along the terrestrial array (Fig. 5). The Hf isotopes in this study are consistent with the Hf data of Fangcheng basalts reported by Yang et al. (2006). Previous studies indicate that the Mesozoic basalts derived from an isotopically enriched mantle source with negative ε Nd , variable 87 Sr/ 86 Sr i , unradiogenic Pb isotope ratios (e.g., Xu, 2001;Zhang et al., 2002;Gao et al., 2008;Yang and Li, 2008). ...
We report here the whole-rock Hf–Nd isotopic data on Mesozoic–Cenozoic basaltic lavas from Liaoning, Shandong, Hebei and Jiangsu–Anhui regions in the North China Craton (NCC). These lavas can be readily subdivided into two groups. The older (N 110 Ma) lavas are relatively depleted in high field strength elements (HFSEs), and have enriched Sr–Nd–Hf isotopic compositions. The younger (b 110 Ma) lavas, by contrast, have OIB-like trace element systematics and more depleted Sr–Nd–Hf isotopic compositions. Among several possibil-ities, the most straightforward explanation is that the N 110 Ma lavas may have largely derived from the "ancient metasomatized lithosphere" or contain contributions from ancient continental crust in the process of lithosphere thinning, whereas the b 110 Ma lavas with more depleted Sr–Nd–Hf isotopic compositions were most likely derived from the asthenosphere when the lithosphere had already been thinned with the ancient lithospheric mantle being removed. We thus propose that the thinning of the ancient cratonic mantle lithosphere beneath the NCC must have been largely completed by ~110 Ma. In this context, we infer that ancient sub-continental lith-ospheric material, once entering the asthenosphere, may be important as an enriched component for intra-plate basaltic magmatism, including ocean island basalts (OIBs). In addition, all these basaltic lavas plot along the terrestrial array in the Hf–Nd isotopic space, suggesting that the mantle source isotopic variation is largely controlled by simple magmatic processes, i.e., low-degree melt metasomatic enrichment (e.g., elevated Hf/Lu and Nd/Sm ratios) and melt extraction-related depletion (e.g., lowered Hf/Lu and Nd/Sm ratios).
... ε Hf = 3.84-6.02). These basalts well plot in the field defined by the Cenozoic basalts from the east NCC (Basu et al., 1991;Liu et al., 1995aLiu et al., , 1995bPeng et al., 1986;Song et al., 1990;Yang et al., 2006;Zhang et al., 2012b;Zhi et al, 1990;Zhu et al., 2012) in both Sr-Nd and Nd-Hf isotope spaces (Fig. 8). ...
... The average ratios of Nb/U and Ce/Pb in OIB and MORB are after Hofmann et al. (1986). Yang et al., 2006;Zhang et al., 2012b;Zhu et al., 2012), and plot in the OIB field in 87 Sr/ 86 Sr i -ε Nd (t) and ε Nd (t)-ε Hf (t) spaces ( Fig. 8; except the sample DB-07 with higher 87 Sr/ 86 Sr i ), suggesting that they may have similar origin, i.e., partial melting of asthenosphere. The observations below can provide further evidence to support this interpretation: (1) Their Ba/Nb and La/Nb ratios range from 2.65 to 11.09 and 0.45 to 0.57, respectively, overlapping the present-day OIB (see Fig. 11a); (2) the Ce/Pb ratios of 23.5-21.9 ...
... The data source for Mesozoic basalts from Fangcheng, Wulanhada high-Mg andesites, Paleozoic kimberlite and peridotite from Mengyin, the Jining 23.5-21.9 Ma basalts and Jining Quaternary basalts are the same as in (a), the Hf isotope data for NCC Cenozoic basalts are fromYang et al. (2006),Zhu et al. (2012), andZhang et al. (2012b). Reference Terrestrial Array (ε Hf = 1.36ε ...
This study shows lithosphere evolution history in the west North China Craton (NCC) from the early Cretaceous to Quaternary by studying the major element, trace element and Sr-Nd-Hf isotope compositions in Jining basalts of 119.6 − 108.6 Ma, 23.5 − 21.9 Ma and 1.3 − 0.11 Ma.
The early Cretaceous basalts (119.6 − 108.6 Ma) display enriched characteristics with high contents of incompatible elements, high 87Sr/86Sri, low εNd(t) and low εHf(t). These basalts resulted from partial melting of ancient metasomatized lithospheric mantle, and we consider the 119.6 − 108.6 Ma magmatism as indicating lithosphere thinning in the west NCC. Although the Pacific slab seen seismically in the mantle transition zone beneath eastern China is no older than 60 Ma, there exists convincing evidence for the presence of the Paleo-Pacific slab in the transition-zone in the Mesozoic. Thus we propose that the water released from the transition-zone slab hydrated the overlying lithosphere and further converted the base of the lithosphere into asthenosphere. This is the most likely mechanism responsible for the lithosphere thinning in the west NCC and the petrogenesis of the Jining 119.6 − 108.6 Ma basalts.
The Jining 23.5 − 21.9 Ma basalts also have high contents of incompatible elements, but they display high εNd(t), high εHf(t) and variably low 87Sr/86Sri. We propose that these Miocene basalts were derived from the asthenosphere with contributions from ancient metasomatized lithospheric mantle during melt ascent. The Jining Quaternary basalts (1.3 − 0.11 Ma) represent the melt of upwelling asthenosphere with low 87Sr/86Sri, high εNd(t) and high εHf(t). Upwelling and decompression melting of the eastward flowing asthenosphere from beneath western plateaus to beneath eastern hilly plains in the Cenozoic is the most plausible mechanism for the petrogenesis of Jining Cenozoic basalts (both of 23.5 − 21.9 Ma and 1.3 − 0.11 Ma), but the Jining 1.3 − 0.11 Ma basalts must have been produced beneath even thinner lithosphere.
Taken together geophysical studies and our petrological and geochemical studies of all these three episodes of the Jining basalts, we propose that the lithosphere in the west NCC has been thinning since the early Cretaceous and the thinning continues to the present.
... For comparison, plotted also are average composition of present-day oceanic island basalts (OIB; Sun and McDonough, 1989), global subducted sediment (GLOSS, Plank and Langmuir, 1998), average crust of NCC (Gao et al., 1998) and the range of >110 Ma Basalts from the ENCC (Dai et al., 2016;Liu et al., 2008). (Dai et al., 2016;Gao et al., 2008;Guo et al., 2003;Guo et al., 2013;Ling et al., 2009;Meng et al., 2015;Yang et al., 2006;Yang et al., 2012;Zhang et al., 2002Zhang et al., , 2003, Early Createcous basalts from WNCC (Guo et al., 2014(Guo et al., , 2018 and Paleozoic kimberlite and mantle xenoliths (Wu et al., 2006 and references therein). Reference Terrestrial Array (ε Hf = 1.36ε ...
We present new bulk-rock 40 Ar/ 39 Ar age, major and trace elements and Sr-Nd-Hf isotopic data on the early Cretaceous intra-plate alkali basalts from the Western North China Craton (WNCC) to study the origin of the metasomatized cratonic lithosphere mantle. The age of these basalts is ~116 Ma. These basalts have elevated incompatible element abundance with high [La/Sm] N (2.80-4.56) and enriched Sr-Nd-Hf isotopic compositions (87 Sr/ 86 Sr i = 0.7062-0.7075, ε Nd (t) =-6.0 to-13.0 and ε Hf (t) =-8.3 to-17.4), being similar to the contemporary analogues from the Western North China Craton and Paleozoic kimberlites and mantle xenoliths. The WNCC basalts also show good correlations between ɛ Nd (t) and ɛ Hf (t), and high [La/Sm] N. All these geochemical observations are consistent with the interpretation that these basalts originated from partial melting of the lithospheric mantle that experienced melt metasomatism. Two types metasomatism melts are required to explain the geochemical characteristics of these rocks. The obvious negative Nb-Ta (compared with K)-Ti and positive Ba-Pb anomalies observed in these basalts further constrain that one of the metasomatic melts was derived from the subducted terrigenous sediment. Furthermore, the overall higher P/Nd, Nb/La and Nb/Th and lower Lu/Hf of basalts in the WNCC suggest that there is also contribution of low-F melts from asthenosphere mantle. Collectively, we suggest that the formation of the metasomatized lithosphere mantle beneath the WNCC is the process of metasomatic reaction between mantle peridotite and the melts of different origin to generate metasomatic veins containing amphibole/phlogopite. Partial melting of the metasomatic lithospheric mantle at 106-120 Ma in the WNCC was considered to be induced by thermal perturbation that was ultimately related to the breakoff of the subducted oceanic slab following the closure of the Mongolia-Okhotsk ocean.
... In situ zircon Hf isotopic analysis was carried out on a Neptune MC-ICP-MS equipped with a Geolas-193 laser ablation system (LAM-MC-ICP-MS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The de- tailed analytical procedures are given by Wu et al. (2006) and Yang et al. (2006b). Hf isotopic analyses were obtained on 30 zircons, including 16 zircons dated at ∼274 Ma by SHRIMP and 14 undated zircons that share similar CL image features to those ca. ...
Zircon U-Pb ages, whole-rock geochemical and zircon Hf isotope data are reported for the Permian A-type quartz syenites-granites in western Tarim Block, Northwest China. Zircon U-Pb dating indicates these alkali plutons were emplaced at 270-277 Ma. Elemental geochemistry shows their typical A-type signatures, i.e., high total alkali, HFSE, REE contents and Ga/Al, FeOtot/(FeOtot + MgO) and Nb/Y ratios. Isotopically, their epsilon Nd values vary from − 2.6 to 2.0. Based on zircon Hf isotope compositions, they could be divided into two sub-groups: Group I exhibits high positive εHf(t) ranging from 3 to 11 (mostly within the range of 5 to 9), which is highly comparable with the syenites in Bachu area located just south of these plutons. Group II has εHf(t) ranging from − 3 to 2. The second group exhibits comparable Nd-Hf isotope compositions with the spatially and temporally related Piqiang ultramafic-mafic complex. Integrating geological, geochemical and Nd-Hf isotope data, we suggest that the A-type syenite-granites were formed via intensive crystal fractionation from a common plume-derived parental mafic magma, coupled with variable extent of crustal contamination. Moreover, the extent of the crustal contamination increase from inner Tarim towards South Tianshan Collisional Belt.
... The predominance of a fertile uppermost SCLM domain across the major tectonic boundaries beneath eastern China is consistent with geophysical data showing a hot and thin SCLM (e.g., Zhao et al., 2009;Zhao et al., 2011) and supports the argument that the cratonic mantle beneath the present-day NCC has be mostly removed and replaced by a hot, thin, and juvenile SCLM (Griffin et al., 1992;Fan and Menzies, 1992). Recent Re-Os isotopic studies Basu et al. (1991), Song et al. (1990), Han et al. (1999), Zhang et al. (2005), Tang et al. (2006), Yang et al. (2006), Choi et al. (2008), Zeng et al. (2011); Hannuoba mantle xenoliths after Song and Frey (1989), Tatsumoto et al. (1992), Xu (2002), Rudnick et al. (2004), Fan et al. (2005), Choi et al. (2008); Pacific and Atlantic MORB, Indian MORB after Barry and Kent (1998), Zou et al. (2000), Chauvel and Blichert-Toft (2001); GLOSS: subducted oceanic sediments after Plank and Langmuir (1998). MORB, OIB, EMI and EMII after Zindler and Hart (1986); Northern Hemisphere Reference Line (NHRL) after Hart (1984). of both mantle sulfides (e.g., Xu et al., 2008;Wang et al., 2009;Yang et al., 2010) and xenoliths (e.g., Reisberg et al., 2005;Chu et al., 2009;Liu et al., 2011) also demonstrate the juvenile nature of the SCLM beneath all of eastern China, including the NCC. ...
The Jining Cenozoic basalt province, located at the northern margin of the North China Craton, is one of the major Cenozoic basaltic provinces in northern China and Mongolia. In order to constrain the petrogenesis of the Jining basalts and its implications for the geodynamic evolution of the region, four basalt sections in Jining province, the Baiyinxiang, Hanqingba, Xin’anzhan, and Chahanmiao sections, were analyzed for Sr–Nd–Pb–Hf isotopic compositions. These basalts are predominantly composed of alkaline basalts and tholeiites. The tholeiites are overlain by the alkaline basalts in all but the Chahanmiao section. In general, the Jining basalts are similar to oceanic island basalts in terms of their incompatible element patterns.
... In situ zircon Hf isotopic analysis was carried out on a Neptune MC-ICP-MS equipped with a Geolas-193 laser ablation system (LAM-MC-ICP-MS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The de- tailed analytical procedures are given by Wu et al. (2006) and Yang et al. (2006b). Hf isotopic analyses were obtained on 30 zircons, including 16 zircons dated at ∼274 Ma by SHRIMP and 14 undated zircons that share similar CL image features to those ca. ...
Zircon laser ablation inductively coupled plasma mass spectrometry U-Pb age and geochemical and Sr-Nd-Hf isotopic data are reported for the Bachu layered intrusive complex (BLIC) in the western Tarim Block and are used to assess the possible presence of a Permian large igneous province (LIP) in the region. The BLIC intrudes the Silurian-Devonian sedimentary rocks, and our U-Pb zircon dating gives a crystallization age of Ma. Rock types of the BLIC 274 2 include pyroxenite, diorite, syenite, and quartz syenite, with a wide range of SiO 2 contents (38.6%–68.7%) and variably high alkalinity (,). They are enriched in Rb, Ba, Th, Nb, Ta, Zr, Na O K O p 1.5%–12.0% K O/Na O p 0.23–0.9 2 2 2 2 Hf, and light rare earth elements. Isotopically, they are characterized by positive whole-rock Nd(t) values (0.25–2.8, mostly above 2.0) and zircon Hf(t) values (5.8–8.9) and low initial 87 Sr/ 86 Sr ratios (0.7035–0.7045). These features suggest that the BLIC was likely formed by crystal cumulation and fractionation (with negligible crustal contami-nation) of alkali basalts derived from an ocean island basalt–like mantle source (i.e., the asthenospheric mantle) in an extensional regime. We suggest that these mid-Permian igneous rocks, in combination with the voluminous coeval basalts and intrusive rocks covering an total area of ca. 250,000 km 2 in the Tarim Block and surrounding regions, constitute an LIP (the "Bachu LIP") and that the BLIC could be the residue of a feeder for this LIP.
... Trends of crustal AFC and mantle AFC are after Stern and Kilian (1996). Data references: Jianguo (JG) late Cretaceous alkali basalt (derived from asthenospheric mantle), Zhang et al. (2003) and Yang et al. (2006); North China ESLM, Chen et al. (2008); Tongshankou (TSK) granodiorite (derived from the Daye ESLM), Li et al. (2008); North Dabie adakitic hornblende quartz-monzonite (I), adakitic granite (II) and non-adakitic granite (III), Xu et al. (2008a); Chituling diorite, Huang et al. (2008) for ε Nd (t) and Zhang (unpublished data) for ε Hf (t); Datong adakitic granite and Luyang non-adakitic granite in the southern Dabie orogen, Zhang (unpublished data). The Hf isotopes for the JG basalt were determined on a bulk sample and those for other rock on zircons using an in-situ technique. ...
Field observation, petrography and geochemistry of mafic enclaves/dikes and their host felsic rocks from the Meichuan pluton are used to propose a geodynamic model for the southern Dabie orogen, central China. The similar Sr–Nd isotopic ratios [ε Nd (t)=−15, (87 Sr/ 86 Sr) i = 0.70550–0.70597] and zircon Hf isotopic ratios [ε Hf (t)=−25 to −16] indicate that the two types of magmatic rocks were formed by coeval felsic and mafic magmas during the Early Cretaceous (132± 2 Ma, zircon U–Pb age). The adakitic signatures of both the felsic and mafic rocks, such as very high Sr (770–1400 ppm), high Sr/Y ratios (40–130), low Y (3.5–21 ppm) and HREE concentrations are supposed to be features of the primary magmas, indicating that both of them were generated by partial melting of basaltic protoliths at great depths (N15 kbar). The distinctive major element compositions of the felsic and mafic primary magmas could be attributable to different melting temperatures and melting degrees. The mafic enclaves/dike have distinctively high concentrations of MgO (4.4–5.8 wt.%), Cr (229– 374 ppm) and Ni (75–163 ppm), indicating a melt-mantle interaction in which olivine is partly consumed while orthopyroxene and/or pyrope are formed under high-pressure. Modeling suggests that 14% of peridotite relative to melt could have been consumed to elevate the Mg # of melt to the observed values (55–60), and that transformation from orthopyroxene to pyrope in mantle peridotite could have decreased the Al 2 O 3 content from 18–19 wt.% in the initial melts to ~15 wt.% in the resultant mafic melts. Moderate negative zircon ε Hf (t) and bulk ε Nd (t) values also suggest contributions from both enriched lithospheric mantle and ancient lower crust. The results can best be explained by assuming that a block of amphibolite-composition lower continental crust was delaminated into the lithospheric mantle, leading to the formation of mafic magmas. This delamination is attributed to lithospheric extension and asthenospheric upwelling along the Yangtze River fault zone in the late Mesozoic which could have heated up the lithospheric mantle underneath the neighboring southern Dabie orogen. The strong input of heat triggered the delamination of the overlying thickened lower crust into lithospheric mantle by weakening the uppermost mantle and resulted in melting reactions in the delaminated crustal block to produce hot (~1100 °C) intermediate adakitic magmas (SiO 2 =55–60 wt.%). The ascent of these hot magmas and their underplating below the lower crust induced the generation of high-silica melts (SiO 2 = ~70 wt.%) at lower temperature (~925 °C or less). Hybridization processes between the two magma types occurred during further ascent and emplacement in the crust, which could have led to the high concentrations of MgO (2.2–3.5 wt.%), Cr (56–226 ppm) and Ni (33–99 ppm) in the host felsic rocks.
Lu-Hf is one of the isotopic dating and geochemical tracing techniques developed very rapidly during the past years. This paper presents a comprehensive review about the history, geochemical applications in petrology and remained problems surrounding the Lu-Hf isotope system. An overall introduction to the presently used sample digestion, mass spectrometric measurement and laser in-situ ablation techniques is also provided. The development of Hf isotopic systematics can be divided into 3 stages of TIMS, hot-SIMS and MC-ICP-MS, but the advent of MC-ICPMS accelerates their applications in petrology. After presenting the foundmantal geochemical behaviour and principles of the Lu-Hf isotope, this paper reviews its applications in petrology in details, including the isotopic dating of the garnet- and apatite-beraing rocks, early continental crustal formation and evolution, nature and petrogenesis of different mantle end-members, identification of various magmatic processes, regional geodynamic evolution and Hf isotopic variance during metamorphism. Finally, we discuss the uncertainties of 176Lu decay constant, Hf isotopic ratios of commonly used standards of solution JMC475 and zircon/baddeleyite, and closure temperature of Hf isotopic systems.
Mafic magmatism occurring at Qianshan in the Hunchun area of eastern Jilin Province, NE China, comprises a wide spectrum of rock types from olivine gabbro, norite to gabbroic diorite. Zircon U-Pb dating results on a gabbroic diorite indicate an Early Permian emplacement age (273 +/- 2Ma, n =12, MSWD = 1.6). These rocks show typical geochemical characteristics to arc tholeiitic basalt, e. g., enrichments in LILEs (Ba and Sr) and LREEs but depletion in HFSEs (Nb, Ta, Zr and Hf), with La/Nb range of 2. 8 similar to 4.8 and Zr/Sm range of 8.0 similar to 22.5. They also have depleted Sr-Nd-Hf isotopic features, spanning an (87)Sr/(86)Sr((i)) range of 0. 70295 similar to 0.70375, an epsilon(Nd)(t) range of +4. 5 similar to +6. 4, and an epsilon(Hf)(t) range of +9. 6 similar to +14. 6, suggesting their derivation from an isotopically depleted mantle reservoir. Combined major, trace element and Sr-Nd-Hf isotope data suggest that the primary magma was derived from a metasomatized mantle wedge, predominantly modified by fluid released from the subducted sediments. This magma then experienced fractional crystallization of olivine and clinopyroxene to form the wide spectrum of rock types. Generation of Qianshan mafic intrusions was genetically associated with subduction of the Paleo-Asian oceanic slab. Our new results indicate that the existence of the Paleo-Asian Ocean lasted even to tile end of Paleozoic in the eastern segment of the Central Asian Orogenic belt.
There are four types of pyroxenes in the Cenozoic alkali basalt at Changle, Shandong Province; phenocryst clinopyroxenes, metrix clinopyroxenes, clinopyroxene megacrysts and homologous orthopyroxene xenocrysts. The coronas, between the clinopyroxene megacrysts and alkali basalt, could be divided into complex coronas and simple coronas. The complex coronas appeared in the outside of larger clinopyroxene megacrysts and could be divided into four zones based on their characteristics of textures and components. The zones of interior, transitional and exterior are composed of clinopyroxenes with different textures and a few melts and metal oxides. The edge zone is composed of clean clinopyroxene without melt and metal oxide. The clinopyroxenes at the coronas display comb texture in the interior zone, sieve texture in the transitional zone and both similar sector-zoned and sieve texture in the exterior. The compositions of the clinopyroxenes, melts and metal oxides at the coronas are controled by the clinopyroxene megacrysts only in the interior zone, and by basaltic magma in other zones. The simple coronas appeare in the outside of samller clinopyroxene megacrysts and are composed of whole clinopyroxene without melt and metal oxide. The coronas of clinopyroxene megacrysts are resulted from the temperature difference between clinopyroxene megacrysts and basaltic magma. If the grain of clinopyroxene megacryst is larger, the time taken to adjust the difference of temperature is longer, then the complex coronas with four zones will form; if the grain is smaller, the simple coronas with edge zone will form as a result. The compositions of homologous orthopyroxene xenocrysts are within the range of compositions of the orthopyroxenes in the lherzolite xenoliths. The clinopyroxene rim or reaction rim surrounding the orthopyroxene xenocrysts are the results of the compositional differnece between orthopyroxene and basaltic magma, similar to the reaction rim texture described by Bowen (1956). All clinopyroxene megacrysts, clinopyroxenes in the lherzolite xenoliths and part of the core of clinopyroxene phenocrysts are located in the "granulites and the inclusions in basalts" field of Aoki and Shiba (1973) in the A1 IV-A1VI diagram, suggesting a relatively high-pressure crystallization. It suggests that part of the core of clinopyroxene phenocrysts come from mantle lherzolite, and they belong to xenocrysts. Clinopyroxenes in the coronas, metrix clinopyroxenes and a majority of clinopyroxene phenocrysts are plotted in the "igneous rocks" field. Based on the estimated crystallization pressure for clinopyroxene megacrysts and clinopyroxenes in the lherzolite xenoliths, we suggest a formation mechanism for the clinopyroxene megacrysts as following: first, the asthenosphere-derived alkali basaltic melts invaded into the lithospheric mantle near the lithosphere asthenosphere boundary and produced clinopyroxene megacrysts, then the melts with a great lot of clinopyroxene megacrysts mixed with the existing magmatic system from mantle lithosphere entraining an abundance of the xenocrysts of clinopyroxenes and orthopyroxenes and lherzolite xenoliths. The mixed alkali basaltic magmas with clinopyroxene megacrysts, xenocrysts and xenoliths ascended from the crustal magma chambers, and erupted at the surface.
This paper first reports the Ar-Ar geochronology of kimberlite-borne phlogopite megacrysts and the Hf isotopic compositions of the diamondiferous kimberlites from the eastern North China Craton. The paper, combined with the Sr-Nd isotopic data, discussed the kimberlite petrogeneses and their tectonic settings. Ar-Ar geochronology of the phlogopite megacrysts demonstrates that both the Mengyin and Fuxian kimberelites have coherent emplacement ages, about 465 ± 2Ma. This is consistent with the previous results of the perivoskite U-Pb age and phlogopite Rb-Sr isochron ages of the kimberlites. The Sr-Nd isotopic data show that both the Mengyin and Fuxian kimberlites have restricted Nd isotopic compositions (ε nd in the range of -0.4 - 0.2 and - 3.4 - - 2.3, respectivey). However, these kimberlites have extremely large Sr isotopic compositions (I sr in the range of - 0.2 - - 28.1 and 0-75, respectively), in which relative to the Mengyin, the Fuxian kimberlites have lower Nd isotopes and much wider Sr isotopes. Hf isotopic results show that individual kimberlite fields from both the Mengyin and Fuxian regions have quite uniform Hf isotopic compositions, similar to the situation for the Nd isotopes, in which the initial 176Hf /177 Hf ratios of the Mengyin kimberlites are in the range of 0.282474 - 0.282416 with corresponding ε Hf values of - 2.37 ∼ - 0.30 and the initial 176Hf /177Hf ratios of the Fuxian kimberlites are of 0.282305 - 0.282369 with corresponding ε Hf values of - 6.29 ∼ - 4.04. This manifests that Mengyin kimberlites have relatively higher Hf isotopic compositions than those of Fuxian kimberlites. In combination with the Sr and Nd isotopic results, it indicates that the difference in Hf isotopic compositions between the Mengyin kimberlites and the Fuxian kimberlites could reflect the difference in proportions of the involvement of the asthenosphere, the lithospheric mantle and the subducted oceanic crust in the origin of the kimberlitic magmas. These Sr-Nd-Hf isotopic features demonstrate that the generation of the Paleozoic kimberlites of the North China Craton was related with the inward subduction of Paleo-oceanic lithosphere from the circum North China Craton towards the interior of the craton.
The Precambrian Sushui Complex in Zhongtiao Mountain is composed principally of Xiyao and Zhaizi TTG gneisses, Henglingguan and Beiyu calc-alkaline granitoids. SHRIMP zircon U-Pb dating for Xiyao TTG gneiss yield the weighted mean 207 Pb/206 Pb age of 2536 ± 8 Ma, revealing that it was formed in the Neoarchean. In-situ zircon Lu-Hf isotopic analyses of Xiyao and Zhaizi TTG gneisses and Henglingguan-Beiyu calc-alkaline granitoids show that the magmatic zircons have positive εHf (t) values, and these εHf (t) values are plotted within the revolution areas of the 2600-3100Ma old continental crust in the age vs. εHf (t) diagrams. The analytical spots of three inherited zircon cores from the Beiyu calc-alkaline granitoid yield a weighted mean 207 Pb/206 Pb age of 2633 ± 84Ma, and the εHf (t) values of these inherited cores of magmatic zircon grains are, -2.0 - +5.6. The characteristics of zircon SHRIMP U-Pb ages and Hf isotope compositions of these TTG gneisses and calc-alkaline granitoids from the Precambrian Sushui Complex indicate that they were possibly formed in the Neoarchean to Paleoproterozoic tectonic settings of Andes-type continent margin, and the magmas could be derived from the partial melting of juvenile crust materials from the depleted mantle and more older continental crust materials.
In-situ Hf isotope analyses have been carried out on zircons from Jurassic-Early Cretaceous intrusions in Shandong Province. It is shown that the inherited zircons, which were formed during the late Archean ( upper intercept age similar to 2.5Ga), have positive epsilon(Hf)(t) (+8 similar to +1), with Hf model ages clustering between 2.6 similar to 2. 8Ga.These Hf isotopic compositions and model ages are very similar to those of mafic lower crustal xenoliths included in the Paleozoic kimerlites in Fuxian, Liaoning province. The Hf model ages are also similar to the Nd model age of the crust in the North China craton. These inherited zircons are therefore interpreted as derived from the mafic lower crust which was underplated during the late Archean time. New zircons, occurring as rim surrounding inherited zircons or as separate crystals, have U-Pb ages of 177 Ma and 132 similar to 126 Ma. Their epsilon(Hf) (t) are negative ranging from - 23 to - 1. It is proposed that Mesozoic intrusions from Shandong were generated as a result of interplay between three components, namely, enriched lithospheric mantle, depleted mantle and crust. Specifically, Jurassic monzonites from Tongshi, which were derived from late Archean lower crust, provide constraint on the Hf isotopic composition of the lower crust component (epsilon(Hf)(t) = - 20); while gabbroic rocks from Yinan, which were originated from the enriched lithospheric mantle, backup the epsilon(Hf)(t) of the enriched mantle composition of -16. The extremely large variation in epsilon(Hf)(t) (-20 similar to - I) in some samples mirrors the progressive participation of depleted mantle components during magma generation. This change in magmatic source resulted from the lithospheric thinning process which was widespread during the late Mesozoic in North China.
The Fuping Complex is situated in the central part of the North China Craton and consists of four major lithological assemblages: Fuping tonalitic-trondhjemitic-granodioritic gneisses, Longquanguan augen gneisses, Wanzi supracrustal assemblage and Nanying granitic gneisses. SHRIMP U-Pb geochronology combined with U-Th and cathodoluminescence (CL) imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the Fuping Complex. CL images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the Fuping Complex. The metamorphic zircons occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores, and are all characterized by nebulous zoning or being structureless, with extremely high luminescence and very low Th contents. These features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning, comparatively low luminescence and high Th and U contents. SHRIMP U-Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic plutons of the Fuping gneisses were emplaced at 2523 ± 14 Ma, 2499 ± 10 Ma and 2486 ± 8 Ma, respectively; whereas the monzogranitic and granitic plutons of the Longquanguan augen gneisses were intruded, respectively, at 2510 ± 22 Ma and 2507 ± 11 Ma. Prismatic and oscillatory-zoned zircons dominate in the pelitic rocks of the Wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks. The concordant and discordant U-Pb ages of 2502 ± 5 Ma and 2507 ± 14 Ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than ∼2507 Ma ago. In addition, a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 ± 5 (1σ) Ma, which may provide a maximum depositional age for the Wanzi supracrustal rocks. SHRIMP results also reveal that granitic magmatism assigned to the Nanying granitic gneisses occured over a protected interval from ∼2077 ± 13 Ma to ∼2024 ± 21 Ma. The nebulously-zoned zircon grains and overgrowth/recrystallization zircon rims from different rocks yielded similar concordant 207Pb/206Pb ages in the range 1875 to 1802 Ma, interpreted as approximating the age of regional metamorphism of the Fuping Complex. Timing of primary zircon crystallization and regional metamorphism of the Fuping Complex is in general agreement with recent U-Pb zircon ion probe results for the Wutai and Hengshan Complexes that bound the Fuping Complex to the northwest. These areas are characterized by the emplacement of major granitoid bodies at around 2.50 Ga to 2.48 Ga ago, deposition of supracrustal rocks in the Paleoproterozoic, intrusion of Paleoproterozoic granitic bodies at 2.1 to 2.0 Ga, and regional metamorphism at 1.875 to 1.802 Ga. These data indicate that the Fuping and Hengshan Complexes do not represent an older crystalline basement to the Wutai Complex, as suggested in previous tectonic models but, together with the Wutai Complex, represent elements of a single late Archean to Paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western North China blocks at ∼1.85 Ga, which resulted in the final assembly of the North China Craton.
The lithospheric mantle beneath the North China Craton changed dramatically in its geophysical and geochemical characteristics from Palaeozoic to Cenozoic times. This study uses samples of Mesozoic basalts and mafic intrusions from the North China Craton to investigate the nature of this mantle in Mesozoic times. Sr–Nd–Pb isotopic data demonstrate that the Late Mesozoic lithospheric mantle was extremely heterogeneous. In the central craton or the Luzhong region, it is slightly Sr–Nd isotopically enriched, beneath the Taihangshan region it has an EM1 character (87 Sr/ 86 Sr i = 0.7050–0.7066; ε Nd(t) = −17– −10), and beneath the Luxi–Jiaodong region, it possesses EM2-like characteristics (87 Sr/ 86 Sr i up to 0.7114). Compositional variation with time is also apparent in the Mesozoic lithospheric mantle. Our data suggest that the old lithospheric mantle was modified during Mesozoic times by a silicic melt, where beneath the Luxi–Jiaodong region it was severely modified, but in the Luzhong and Taihangshan regions the effects were much less marked. The silicic melt may have been the product of partial melting of crustal materials brought into the mantle by the subducted slab during the formation of circum-cratonic orogenic belts. This Mesozoic mantle did not survive for a long time, and was replaced by a Cenozoic mantle with depleted geochemical characteristics.
Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalts occur as volcanic channel phases with well-developed columnar jointings and contain few spinel lherzolite and pyroxenite xenoliths. They are poor in silica and rich in alkalis, Ti and Al, belonging to alkaline basalts. In trace element compositions, Jianguo basalts are moderately enriched in LREE and LILE, but not depleted in HFSE. They have low Sr and high Nd and Pb isotopic ratios. These geochemical characteristics suggest that Jianguo basalts originated from the depleted asthenosphere, representing an undifferentiated and uncontaminated primitive magma. Presence of these basalts indicates that the lithosphere beneath the region had thickness less than 65 km at the time of basalt eruption and was mainly composed of fertile pargasite-bearing spinel lherzolite and plagioclase pyroxenite. The voluminous basaltic-andesitic magmatism during the early Jurassic-late Cretaceous time indicates that the commencement and accomplishment of lithosphere thinning in the NNCC was much earlier than that in the southern margin, since the mafic-intermediate volcanism only occurred at the Cretaceous time in the southern margin and the basalts with an asthenosphere isotopic signature at the Tertiary. This shows that highly spatial and temporal heterogeneity existed in the Mesozoic lithosphere evolution.
Zircon megacrysts represent a late stage in the crystallisation of the magmas that produced the low-Cr megacryst suite (Ol+Opx+Cpx+Gnt+Ilm+Phl+Zir) found in many kimberlites, and may carry information on the sources of the parent magmas and the interaction of these magmas with the cratonic lithosphere. The isotopic composition of Hf has been measured in 124 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites, using a laser-ablation microprobe (LAM) and a multi-collector (MC) ICPMS. The zircons range in age from 90 Ma to ca 2500 Ma, allowing indirect analysis of mantle-derived Hf over a long time span. Most values of εHf fall between 0 and +10, but zircon suites from several kimberlites range down to εHf = −16. Combined with published Nd data on the silicate members of the low-Cr megacryst suite, these data indicate crystallisation of zircon from magmas lying well below the terrestrial εHf-εNd array. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived peridotite xenoliths suggest that cratonic lithospheric mantle has Hf/Nd (0.3–0.5) greater than estimated Bulk Silicate Earth. The depleted and metasomatised lherzolites and harzburgites that make up much of the Archean lithospheric mantle have Lu/Hf ratios (≤0.15) low enough to account for the lowest εHf observed in the zircons, over time spans of 1–3.5 Ga. We therefore suggest that the magmas from which the kimberlitic zircons crystallised were derived from Depleted Mantle or OIB-type sources, and developed negative εHf through reaction with the subcontinental lithospheric mantle.
In eastern China Palaeozoic kimberlites and Cenozoic basalts have been erupted through the same Archaean crust, thus providing deep probes of the cratonic lower lithosphere over a period of 400 Ma. While Palaeozoic diamondiferous kimberlites point to the existence of thick, refractory lower lithosphere in the east, Cenozoic basalt-borne xenoliths reveal the presence of hot, thin, less refractory lower lithosphere. Remnants of the Archaean lithosphere may have survived as harzburgites which are chemically similar to those from the Kaapvaal craton but very different from recently accreted lherzolites. In the absence of convincing evidence for supra-subduction or intraplate processes it is believed that the dramatic change of lithosphere architecture in the Phanerozoic was caused by indentor tectonics resulting from the collision of India and Eurasia. Passive reactivation and remobilization of the Archaean lower lithosphere, in particular metasome horizons, contributed to Cenozoic magmatism aligned along major lithospheric faults.
We report new Hf (and Nd) data for more than 100 sedimentary samples, recent to Archean in age, from a wide range of depositional environments. These data document the behavior of Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system. In conjunction with existing data for mantle-derived rocks, we now have reasonable constraints on coupled Hf–Nd isotopic behavior in the crust and mantle. Lu/Hf and Hf isotopic compositions are strongly fractionated between muds and sands in passive margin sediments due to concentration of low Lu/Hf, low 176Hf/177Hf, Hf-rich zircons in mature sands. In active margin settings, Lu–Hf fractionation due to the `zircon effect' is minor due to the less weathered and more juvenile character of the sediments. Nd isotopic compositions are not highly fractionated by sedimentary sorting because heavy minerals, also rich in REEs, do not fractionate Sm–Nd efficiently. The lack of a large and systematic fractionation at active margins means that no significant Hf–Nd decoupling occurs here. This is important because sediments at active margins are the most likely to be recycled to the mantle. Hf–Nd isotopic data for all terrestrial samples fall along a single coherent trend (εHf=1.36εNd+2.95) which we call the `terrestrial array'. This array is composed of two complementary components: a mantle array (εHf=1.33εNd+3.19, defined by all oceanic basalts; and a crustal array (εHf=1.34εNd+2.82), defined by sediments, continental basalts, granitoids, and juvenile crustal rocks. The similarity of the crustal and mantle arrays indicates that no large-scale Hf–Nd decoupling occurs between the crust and mantle. The coherency of the terrestrial Hf–Nd array implies mixing within the mantle, due to stirring processes, and also within the crust, due to homogenization by collective sedimentary processes. In addition, tight Hf–Nd covariation may also imply that efficient crust to mantle recycling has modulated isotopic correlation in the silicate Earth. All Hf–Nd arrays, including the terrestrial array, lie significantly above (2–3 εHf units) the BSE (bulk silicate Earth) reference. This would appear to require a hidden reservoir in the Earth, heretofore unsampled, to balance the Hf–Nd isotopic composition of the terrestrial array. However, the discrepancy between the terrestrial array and BSE may simply be due to differences in the way the CHUR (chondritic uniform reference) values were determined for the Lu–Hf and Sm–Nd isotope systems.
A new digestion procedure and chemical separation technique has been developed for measurement of Lu/Hf and Hf isotope ratios that does not require high‐pressure bombs or use of HF or HClO 4 acids. Samples are digested in dilute HCl or HNO 3 after flux‐fusion at 110 0 °C in the presence of lithium metaborate. High field strength elements (HFSE) and rare earth elements (REE) are separated from this solution by co‐precipitation with iron hydroxide. The dissolved precipitate (in 2 mol l ⁻¹ HCl) is loaded directly onto a standard cation exchange column which separates remaining sample matrix from the heavy REE (Lu+Yb), and the middle‐light REE and HFSE (Hf). The middle‐light REE and individual HFSE are then separated (10.5, 9 and 6 mol l ⁻¹ HCl) using a miniaturized column containing TEVA spec resin which provides a REE‐, Ti‐ and Zr‐free Hf cut. This chemical separation scheme can also be readily adapted for isotopic analysis of the Sm‐Nd system and/or the other HFSE (Ti, Zr). Total procedural blanks for this technique are < 10 0 pg and < 2 pg for Hf and Lu, respectively, even when digesting large (0.5 g) samples.
We present data from replicate digestions of international rock reference materials which demonstrate this technique routinely reproduces Lu/Hf ratios to < 0.2% (2s) and ¹⁷⁶ Hf/ ¹⁷⁷ Hf isotope ratios to < 30 ppm (2s). Moreover, the technique is matrix‐independent and has been successfully applied to analysis of diverse materials including basalts, meteorites, komatiites, kimberlites and carbonatites. The relative simplicity of this technique, coupled with the ease of digestion (and sample‐spike equilibration) of large difficult‐to‐dissolve samples, and the speed (2 days) with which samples can be digested and processed through the chemical separation scheme makes it an attractive new method for preparing samples for Lu‐Hf isotopic investigation.
Major- and trace-element as well as Sr–Nd–Pb isotopic data of the Mesozoic Fangcheng basalts provide an insight into the nature of their mantle source and the secular evolution of the lithospheric mantle beneath the North China Craton. Fangcheng basalts include alkali basalt and olivine tholeiite, both characterized by high Mg (Mg#=65–72), Si, and Ca and low K+Na, Ti, and P. They are extremely enriched in LREEs ((La/Yb)N=39.3–49.3) and LILEs (Ce, Rb, Ba, U, Th) and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with slightly negative Pb anomaly. Correspondingly, these basalts are exceedingly high in ε
Sr (74.0~81.5) and low in ε
Nd (–13.1~–14.2) and 206Pb/204Pb (<17.8). Since crustal contamination during the magma ascent is insignificant, the Fangcheng basalts could reflect the nature of its mantle source. The isotopic data of these basalts cannot be explained by mixing of typical mantle components, but can be accounted for by interaction of an old lithospheric mantle with the lower/middle crust. Therefore, we consider that these basalts originated from the Mesozoic lithospheric mantle, which evolved from its Paleozoic counterpart through extensive interaction with a crust-derived melt. We propose that this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. This peculiar Mesozoic lithospheric mantle somehow was in turn replaced later by the hot and thin Cenozoic lithospheric mantle.
During the Late Mesozoic and Cenozoic, extension was widespread in Eastern China and adjacent areas. The first rifting stage spanned in the Late Jurassic–Early Cretaceous times and covered an area of more than 2 million km2 of NE Asia from the Lake Baikal to the Sikhot-Alin in EW direction and from the Mongol–Okhotsk fold belt to North China in NS direction. This rifting was characterized by intracontinental rifts, volcanic eruptions and transform extension along large-scale strike–slip faults. Based on the magmatic activity, filling sequence of basins, tectonic framework and subsidence analysis of basins, the evolution of this area can be divided into three main developmental phases. The first phase, calc-alkaline volcanics erupted intensely along NNE-trending faults, forming Daxing'anling volcanic belt, NE China. The second phase, Basin and Range type fault basin system bearing coal and oil developed in NE Asia. During the third phase, which was marked by the change from synrifting to thermal subsidence, very thick postrift deposits developed in the Songliao basin (the largest oil basin in NE China).
A combined paleomagnetic and geochronologic study has been conducted on an andesite lava sequence at Jianguo (Liaoning province, northeastern China). Thermal demagnetization and thermomagnetic analysis revealed that natural remanent magnetization is carried by both magnetite and hematite. Stepwise thermal demagnetization up to 675°C isolated well-defined reverse characteristic remanent magnetization (ChRM) in three time-independent lava flows with a mean direction of D/I=179.2°/−59.7° with α95=3.0°. It also showed that a high-temperature component (>585°C) has the same ChRM direction as that of the low- to medium-temperature (ca. 170/300–585°C) components. A modified version of the Thellier–Thellier paleointensity method [Coe, J. Geophys. Res. 72 (1967) 3247–3262] with systematic partial thermoremanent magnetization checks was used for paleointensity determinations. Twelve out of 39 samples yielded reliable results in the temperature interval of 170–550°C. Virtual dipole moment values range from 3.9 to 4.7×1022 Am2 with an average of 4.2±0.1×1022 Am2. 40Ar/39Ar age determination on one lava flow is 116.8±3.0 Myr (2σ error, relative to Fish Canyon sanidine: 28.02±0.28 Myr). The magnetic reversed polarity with well-defined age could correspond to the ‘ISEA’ within the older part of the Cretaceous normal superchron (CNS), but we cannot rule out the possibility that this reversal interval could also correspond to the M0r due to many ambiguities on previous published ages on ISEA and M0r intervals as well as the new monitor age correction we applied. It also suggests that a weak magnetic field nature already documented before the CNS extended at least into the very beginning of the CNS.
Various types of eclogite, including coesite-bearing varieties formed under high-P-high-T conditions (up to 27 kbar, 700–800°C), and glaucophane schist occur in the Dabie Mountains and the SuLu terrane, eastern China. Dating of these high-pressure rocks by the SmNd mineral isochron and method suggests that the occurrence of the high-pressure metamorphism was during the early Triassic. LREE-enriched chondrite-normalized patterns for type-II eclogites and low initial ϵNd of − 14 to − 3 and the (0.706–0.710) for various types of eclogites suggest that their protoliths were mainly derived from Precambrian island arc or intraplate basalts in the basement of the Yangtse Block and the enriched pyroxenite layer in alpine peridotite. The P-T-t path of eclogite from the southern Dabie Mountains suggests that the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) of the continental crust; (2) later gentle uplift with rise of the Dabie Mountains in the late Jurassic and Cretaceous (at 134 Ma). It is proposed that the collision between the North China Block and Yangtse Block began in the late Permian or early Triassic with a north-dipping subduction zone. This was followed by subduction of the continental crust of the Yangtse Block under the North China Block during the Triassic. These two continental blocks were welded into a single tectonic unit in the late Jurassic or early Cretaceous. Different cooling histories for a coexisting gneiss and eclogite pair from Shima area suggest that the eclogite and their country rocks are not always coherent to each other. Some of them may have been juxtaposed through tectonic processes from different levels.
Geochemical and isotopic data from Mesozoic lavas from the Jianguo, Niutoushan, Wulahada, and Guancaishan volcanic fields on the northern margin of the North China Craton provide evidence for secular lithospheric evolution of the region. Jianguo lavas are alkaline basalts with LILE- and LREE-enrichment ((La/Yb)N=12.2–13.2) and MORB-like Sr-Nd-Pb isotopic ratios ((87Sr/86Sr)i<0.704; εNd=3.9–4.8; (206Pb/204Pb)i≈18). Niutoushan basalts are similar but show evidence of olivine fractionation. Wulahada lavas are high-Mg andesites (Mg#∼67) with EM1 Sr-Nd-Pb isotopic signatures. Geochemical data suggest that the basalts originated from MORB-type asthenosphere whereas the high-Mg andesites were derived an EM1 mantle source, i.e., a refractory lithospheric mantle modified by a previously subducted slab. The result, combined with the available data of the Mesozoic basalts from the southern portion of the NCC (Zhang et al., 2002), manifests a vast secular evolution of the lithospheric mantle beneath the eastern NCC from the Paleozoic refractory continental lithosphere to this Mesozoic modified lithosphere. Compared with the cratonic margin, the lithospheric mantle beneath the center of the craton was less extensively modified, implying the secular evolution was related to the subduction processes surrounding the NCC. Therefore, we suggest that the interaction of the slab-derived silicic melt with the old refractory lithospheric mantle converted the Paleozoic cratonic lithospheric mantle into the late Mesozoic fertile mantle, which was also different from the Cenozoic counterpart. A geodynamic model is proposed to illustrate such a secular lithosphere evolution.
Early Cretaceous Fangcheng basalts, erupted in Luxi region of Shandong Province, China, contain olivine xenocrysts with clear compositional zonations, which provide evidence for important mantle-melt reactions. These zoned olivines are fine-grained (200∼900 μm) and the core of the relatively larger grains have compositions (such as Mg# = 88∼92) similar to those of olivines from the mantle peridotitic xenoliths entrained in Cenozoic basalts from the Sino-Korean craton and their rims (Mg# = 76∼83) are compositionally close to those of the olivine phenocrysts from the host basalts. These compositional features as well as textural characteristics such as rounded and embayed crystal shape, well-developed cracks and grain sizes demonstrate that these olivines are mantle xenocrysts disaggregated from the lithospheric peridotites. The zoned texture was formed through rapid reaction between olivine and host melt. This may suggest that mantle-melt reaction was once very significant in the Mesozoic lithospheric mantle beneath the southeastern portion of the Sino-Korean craton, which we consider to be responsible for the replacement of lithospheric mantle from the Paleozoic refractory (high-Mg) peridotitic mantle to the late Mesozoic fertile (low-Mg) and enriched mantle with the loss of more than 120 km Archaean lithospheric keel in the region.
Major and trace element and Nd–Sr isotope data of the Mesozoic Laiwu–Zibo carbonatites (LZCs) from western Shandong Province, China, provide clues to the petrogenesis and the nature of their mantle source. The Laiwu–Zibo carbonatites can be petrologically classified as calcio-, magnesio- and ferro-carbonatites. All these carbonatites show a similarity in geochemistry. On the one hand, they are extremely enriched in Ba, Sr and LREE and markedly low in K, Rb and Ti, which are similar to those global carbonatites, on the other hand, they have extremely high initial 87Sr/86Sr (0.7095–0.7106) and very low εNd (−18.2 to −14.3), a character completely different from those global carbonatites. The small variations in Sr and Nd isotopic ratios suggest that crustal contamination can not modify the primary isotopic compositions of LZC magmas and those values are representatives of their mantle source. The Nd–Sr isotopic compositions of LZCs and their similarity to those of Mesozoic Fangcheng basalts imply that they derived from an enriched lithospheric mantle. The formation of such enriched lithospheric mantle is connected with the major collision between the North China Craton (NCC) and the Yangtze Craton. Crustal materials from the Yangtze Craton were subducted beneath the NCC and melts derived from the subducted crust of the Yangtze Craton produced an enriched Mesozoic mantle, which is the source for the LZCs and Fangcheng basalts. The absence of alkaline silicate rocks, which are usually associated with carbonatites suggest that the LZCs originated from the mantle by directly partial melting.
The geologic framework of the Qinling orogen was built up through interplay of three blocks, the North China block (including the North Qinling), the South Qinling, and the South China block, separated by the Shangdan and Mianlue sutures. The Shangdan suture resulted from Middle Paleozoic collision of the North China block and the South Qinling. The Mianlue suture resulted from Late Triassic collision of the South Qinling and the South China block. Present upper crust of the Qinling is structured dominantly by thrust–fold systems. The North Qinling displays thick-skinned deformation with crystalline basement involved, whilst the South Qinling is characterized by thin-skinned thrusts and folds detached above the Lower Sinian. Two types of Precambrian basement, crystalline and transitional, are defined according to lithology and metamorphic grade and different in age. Stratigraphic and sedimentary architecture is characterized by distinct zonation.
A novel method for the single-step separation of Zr + Hf from all matrix elements of geological samples has been developed for Hf isotopic measurements using multiple collector-ICP-mass spectrometry. The method combines an effective sample decomposition by LiBO2 fusion with a selective separation of Hf + Zr by a solid-phase extraction material based on dipentyl pentyl phosphonate, commercially available as U-TEVA.Spec. Using this simple and rapid procedure, Hf and Zr can be isolated in a single separation step with good recoveries (>90%) and satisfactory blank levels (approximately 55 pg of Hf), so that a subsequent isotopic measurement with ICPMS is possible. An excellent separation from rock-forming constituents is achieved, including those elements (Al, P, Ti, Cr, Fe, Mo, etc.) known to interfere in conventional separation methods based on ion-exchange techniques. The potential of this new method for Hf isotopic analysis is demonstrated by replicate MC-ICPMS measurements of 176Hf/177Hf ratios in seven international reference materials of silicate rocks, spanning a range of Hf contents and bulk compositions.
A combined procedure for separating Lu, Hf, Sm, Nd, and rare earth elements (REEs) from a single sample digest is presented. The procedure consists of the following five steps: (1) sample dissolution via sodium peroxide sintering; (2) separation of the high field strength elements from the REEs and other matrix elements by a HF-free anion-exchange column procedure; (3) purification of Hf on a cation-exchange resin; (4) separation of REEs from other matrix elements by cation exchange; (5) Lu, Sm, and Nd separation from the other REEs by reversed-phase ion chromatography. Analytical reproducibilities of Sm-Nd and Lu-Hf isotope systematics are demonstrated for standard solutions and international rock reference materials. Results show overall good reproducibilities for Sm-Nd systematics independent of the rock type analyzed. For the Lu-Hf systematics, the reproducibility of the parent/daughter ratio is much better for JB-1 (basalt) than for two analyzed felsic crustal rocks (DR-N and an Archaean granitoid). It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.