REE distribution and H-O-Li stable isotopes of lower crustal granulite minerals

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The North China Craton

The North China Craton (NCC) is the Chinese part of the Sino-Korean craton, named sometimes the North China Block in some literatures. It is one of the most ancient cratons on the Earth, composed of early Archean and Proterozoic metamorphic rocks with the oldest recorded crustal ages >3.8 Ga (e.g. Liu et al., 1992), and is the largest craton in China, covering an area of >1,700,000 km2. It is separated from the Mongolian Block by the eastern Central Asian Orogenic Belt in the north, and from the Yangtze Craton, part of the South China Craton, by the Triassic Dabie-Sulu UHP belt in the south (Fig. 2-2). The NCC is crosscut by two large-scale geophysical and geological linear zones. To the west, it is cut by the north-south trend Daxing’anling-Taihangshan Gravity Lineament (DTGL), which finally separates two topographically and tectonically different regions and is probably related with the diachronous lithospheric thinning of the craton (Ma, 1989; Xu, 2007); to the east, it is traversed by the Tan-Lu Fault Zone (TLFZ), which is associated with significant Cenozoic and Mesozoic volcanism.
Based on chronological ages, lithological assemblages, tectonic evolutions and P-T-t paths of metamorphic rocks, the NCC can be divided into three areas: the Eastern and Western Blocks and the intervening Central-Orogenic Belt (Fig. 2-2: Zhao et al., 1998a; Zhao et al., 2000, 2001b). The Eastern Block consists mainly of 3.5-2.5 Ga orthogneisses (dominated by tonalitic, trondhjemitic and granodioritic gneisses (TTGs)), 2.5 Ga granitoids and less amounts of ultramafic to mafic volcanics and sedimentary supracrustal rocks including banded iron formations. The Western Block is composed predominantly of late Archean TTG gneisses, together with ultramafic to felsic volcanic rocks metamorphosed at greenschist to granulite facies, and the Paleoproterozoic khondalite series and 2.5 Ga S-type granites. The Central Orogenic Belt is made up primarily of late neous sedimentary rocks interleaved with thin basalt flows. The collision be-tween the Eastern and Western Blocks may have led to the formation of the Central Orogenic Belt and the final amalgamation of the NCC.
There are contrasting views towards the timing of the collision between the Eastern and Western Blocks. Multi-grain zircon U-Pb age populations from TTG gneisses of the Central Orogenic Belt have upper intercept of 2.5-2.7 Ga and lower intercept of 1.8-2.0 Ga. These younger ages are consistent with Sm-Nd ages of garnets from the high-pressure granulites in this belt and 40Ar/39Ar ages of hornblendes in amphibolites and biotites in TTGs, along with SHRIMP zircon rim ages of the TTGs and supracrustal rocks (Zhao et al., 2000, 2001b). The age of 1.8-2.0 Ga are interpreted as the age of metamorphic overgrowth. These, as well as near-isothermal decompressional clockwise P-T paths, led Zhao et al. (2000; 2001b) to propose collision occurred between the two Blocks at ~1.8 Ga. However, a recent report of a 2.5 Ga ophiolite complex in the northern Central Orogenic Belt implies a much older collisional event between these two blocks (Kusky et al., 2001). Li et al. (2000; 2001b) suggested a model that combines all these observations and assumes collision occurred between the two blocks at ~2.5 Ga followed by rifting during the 2.3-2.4 Ga with subsequent collision at 1.8-2.0 Ga representing the final amalgama-tion/cratonization event.
The NCC experienced widespread tectonothermal reactivation, beginning from the early Paleozoic but mainly during the late Mesozoic and Cenozoic, as manifested by the emplacement of early Paleozoic kimberlites, voluminous late Mesozoic basaltic rocks and granites and Cenozoic alkali basalts (Fan et al., 2000; Zhang et al., 2002; Yang et al., 2003; Zhang et al., 2004; Wu et al., 2005). The NCC also experienced development of extensive sedimentary basins (most of the eastern portion of the craton is covered by Quaternary sediments) and presently has higher heat flow (60 mW/m2: Hu et al., 2000) compared to other Archean and Proterozoic cratons (Nyblade et al., 1990; Jaupart and Mareschal, 2003). The changes in tectonic/magmatic activity are also reflected in a change in the type and composition of mantle xenoliths. Xenoliths carried in Ordovician kimberlites are deep-seated garnet-facies peridotites. These xenoliths, together with the appearance of diamonds in the kimberlites, indicate a cool geotherm, characterized by low heat flow of 36-40 mW/m2, and thick lithospheric keel (~ 200 km). By contrast, xenoliths carried in Cenozoic basalts are dominated by fertile spinel peridotites, which record shallower and hotter lithospheric mantle, in good agreement with an average present-day surface heat flow of 60 mW/m2 and thin lithosphere of 60-80 km. Collectively, these observations indicate the removal of about 80-140 km of Archean lithosphere from beneath the eastern NCC (Fig. 2-3).
(after Xu (2001). OB+AOB: subalkali basalts; ALK+BA+NE: alkali and strongly alkali basalts)
The lithospheric processes, such as diachronous lithospheric thinning, and structures were (or are) probably different between different regions beneath the NCC, separated especially by the DTGL. Comparison of Cenozoic basalts, and xenoliths enclosed in them, from contrasting regions reveals that the DTGL is not only a physical but also a chemical boundary that divides the NCC into two different lithospheric domains. Crustal elevation, morphology, chemical composition, crustal and lithospheric thickness, and gravity anomaly all change remarkably across the DTGL (Xu, 2007). The evolution and composition of both basalts and the captured xenoliths, in terms of the 144Nd/143Nd and 187Os/188Os ratios, imply that the lithosphere is in progressive thinning in the western NCC, and in progressive thickening in the eastern NCC during the Cenozoic (Fig. 2-4: Xu et al., 2004a). Such contrasting lithospheric processes may be related with diachronous extension in the NCC, with initial extension in the eastern part due to the late Mesozoic Paleo-Pacific subduction and subsequent extension in the western NCC induced by the early Tertiary Indian-Eurasian collision (Xu et al., 2004a).

The Dabie- Sulu UHP Belt

The Dabie-Sulu UHP belt lies between the North and South China Cratons, extending from east to west for ca. 2000 km in the central-eastern China (Fig. 2-2). It is separated into two terranes by about 500 km of left-lateral strike-slip displacement along the TLFZ. The Sulu terrain in the east is segmented into a number of blocks by several NE-SW trending faults subparallel to the TLFZ, and the Dabie terrain in the west is the major segment bounded by the TLFZ to the east and separated into a series of continuous zones by several EW-trending faults of large scales. The formation of the Dabie-Sulu UHP belt proceeded mainly in the Triassic, caused by collision between the North China and Yangtze Cratons with peak metamorphism at ~ 245 Ma (Hacker et al., 1998).
The basement of the Dabie-Sulu UHP terranes is metamorphic and igneous, such as schists, greenstones, gneisses, and rare quartzites, marbles, granulites, and eclogites, intruded by granitoids. The occurrence of eclogites first suggests that pressures of metamorphism were high. Discovery of coesite, diamond, and extreme 18O-depletion, as well as exsolution of clinopyroxene, rutile and apatite, in eclogites (e.g. Okay et al., 1989; Wang et al., 1989; Xu et al., 1992; Yui et al., 1995; Zheng et al., 1996; Ye et al., 2000) demonstrates that deep subduction of continental crust to mantle depths of about 200 km and the subsequent quick exhumation (see also a review by Zheng et al., 2003).
The chronological evolution in terms of the subduction and exhumation of the Dabie-Sulu UHP Belt can be simplified by the following (Fig. 2-5): the peak metamorphism occurred at ~ 240-245 Ma, and it experienced eclogites-facies recrystallization at ~ 225-215 Ma and amphibolite-facies retrogression at ~ 200 Ma during the exhumation to crustal levels (Zhao et al., 2006; and references therein).

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The South China Craton

The South China Craton (SCC), or South China Block in some literatures, is composed of the Yangtze Craton and the Cathaysia Craton (Fig. 2-2). It was formed by the amalgamation of a collage between the two cratons during the Grenvillian-age orogeny (Chen et al., 1991; Shu and Charvet, 1996; Li et al., 2002). The Yangtze and Cathaysia Cratons are bounded by the Jiangshao fault zone in the east region, and usually by the south edge of the Jiangnan orogeny belt, exampled by some Proterozoic exposures such as Banxi Group, Lengjiaxi Group and/or Shuangqiaoshan Group, in the middle and west-south regions.
The Yangtze Craton is one of the three oldest continental blocks in China. The recent advances, especially some chronological work, greatly improved our knowledge about the geological and tectonic evolution of this craton. These include mainly the following several aspects: (1) The existence of a widespread Archean basement with ages up to >3.2 Ga, as indicated Sm-Nd and Hf model ages and SHRIMP zircon U-Pb ages (Gao et al., 1999; Qiu et al., 2000; Zhang et al., 2006; Zheng et al., 2006b). The most ancient exposed rocks are found in the west and north edges of this craton, exampled remarkably by Kangding Group in west Sichuan Province and Kongling Group in west Hubei Province (Fig. 2-6). (2) Paleo- to Meso-Proterozoic magmatisms are widely distributed in the south edge of this craton (Zhang et al., 2006; Zheng et al., 2006b), indicating the wide presence of Proterozoic basement. (3) Neo-Proterozoic mafic and granitic magmatisms are widely found in the margins of this craton (Li, 1999; Zhou et al., 2002a; Li et al., 2003a; Li et al., 2003b; Zheng et al., 2004b), but the age-populations are markedly different, e.g. 720-770 Ma in the north margin (Zheng et al., 2004b) vs. 820-830 Ma in the others. (4) This craton possesses abundant ore deposits, especially rare metals, such as gold, copper, etc., in its central and east regions (e.g. Zhai et al., 1996).
The Cathaysia Craton is a relatively young craton compared to the NCC and the Yangtze Craton. The basement of this craton is dominantly Paleo- to Meso-Proterozoic, with some local late Archean component (Chen and Jahn, 1998). The exposed terrains in this craton are mainly sedimentary rocks, derived from variable time-periods, and magmatic rocks dominated by Mesozoic granitites. Precambrian basement rocks in this craton are exposed mainly in an area between the Phanerozoic Jiangshao and Lishui-Haifeng faults in northwestern Fujian and southwestern Zhejiang Provinces (Fig. 2-6). Late Meso-Proterozoic igneous rocks have been recently documented in southwest of this region, e.g. southern Jiangxi and northeastern Guangdong Provinces (Xie et al., 2001), implying a possible southwestern extension of the basement. The Precambrian basement in this craton has been divided into two subunits based on lithologic and structural features and metamorphic grades (Li, 1997): (1) The older unit, named the Mayuan complex in northwestern Fujian and the badu complex in southwestern Zhejiang, consists mainly of leptynites, schists, gneissic granites and mafic and felsic meta-volcanic rocks. All these rocks were metamorphosed to amphibolite-facies at 4-6.5 kbar and 550-650 °C (Jin and Sun, 1997). The formation age of this unit is about 1.7-1.9 Ga by zircon U-Pb method (Gan et al., 1993; Li, 1997), however, there are as yet no metamorphic ages available for this unit. (2) The younger unit, named the Mamianshan Group in northwestern Fujian and the Longquan Group in southwestern Zhejiang (Fig. 2-6), consists mainly of micaceous-/quartz-schist and micaceous leptynites, meta-volcanic rocks, and banded Fe-bearing marbles and quartzites. These rocks underwent intense deformation, and were metamorphosed to upper-greenschist to lower-amphibolite facies at 3-4.5 kbar and 530-600 °C (Jin and Sun, 1997). The formation age of this unit is about 818 Ma from SHRIMP U-Pb dating on zircon in rhyolite (Li et al., 2005).
After the collision, or amalgamation, of the Yangtze and Cathaysia Cratons, a failed rifting was developed roughly along the suture between them during the Neoproterozoic and Paleozoic, geographically through Hunan, Jiangxi, west Guangdong and east Guangxi Provinces. The rifting was manifested by the >13 km thick deposition of Neoproterozoic to Paleozoic abyssal marine carbonatitic / clastic sequence in the center and the 2-5 km shallow-sea carbonate deposits in the rifting margins (Wang and Li, 2003). These pre-Mesozoic sequences were overprinted by the Indosinian tectonothermal events and unconformably overl-ain by the late Mesozoic terrestrial clastics. The Indosinian mafic magmatism, ~ 224 Ma, is missing except for some gabbroic xenoliths hosted by Mesozoic basalts in Daoxian, southern Hunan Province (Guo et al., 1997b).

Lithospheric thinning in eastern China

The lithospheric thinning in eastern China is a warmly and widely discussed topic in the last decade (Fan and Menzies, 1992; Menzies et al., 1993; Griffin et al., 1998; Menzies and Xu, 1998; Menzies et al., 2007). It is not clear, however, about its scale, mechanism and timing, as well as tectonic controlling factors, for this geodynamical process.

Table of contents :

1 Introduction
1.1 The continental lower crust
1.1.1 Water in the deep crust
1.1.2 Water in nominally anhydrous minerals
1.1.3 H-O-Li stable isotopes
1.2 Framework of this study
1.3 Aim of this study
1.4 Structure of this study
2 Geological background
2.1 The North China Craton
2.2 The Dabie-Sulu UHP Belt
2.3 The South China Craton
2.4 Lithospheric thinning in eastern China
3 Localities and samples
3.1 Hannuoba
3.2 Nushan
3.3 Daoxian
3.4 Samples
3.4.1. Hannuoba xenolith granulites
3.4.2. Hannuoba terrain granulites
3.4.3. Nushan xenolith granulites
3.4.4. Daoxian xenolith granulites
3.4.5. Hannuoba xenolith peridotites
3.4.6. Nushan xenolith peridotites
4 Analytical methods
4.1 Petrography and sample documentation
4.2 Electron microprobe (EMP)
4.3 Fourier transform infrared spectroscopy (FTIR)
4.3.1 Spectrometer and measurement parameters
4.3.2 Calculation of water content
4.4 Isotopic ratio mass spectrometry (IRMS)
4.5 Secondary ion mass spectrometry (SIMS)
4.5.1 Rare earth elements
4.5.2 Hydrogen isotopes
4.5.3 Oxygen isotopes
4.5.4 Lithium contents and isotopic compositions
5 Results
5.1 EMP results
5.1.1 Granulites
5.1.2 Peridotites
5.2 FTIR results
5.2.1 Granulites Near-IR absorption Hydrogen-related species Water content
5.2.2 Peridotites H-related species Water content
5.3 IRMS results
5.4 SIMS results
5.4.1 REE contents
5.4.2 Hydrogen isotopic compositions
5.4.3 Oxygen isotopic compositions
5.4.4 Lithium contents and isotopic compositions
6 Water in the continental lower crust
6.1 Preservation of initial hydrogen information
6.2 Distribution of water within sub-grain scale
6.3 Partitioning of water between lower crustal phases
6.4 Water budget in the lower crust
6.5 Water content contrast between Precambrian and Phanerozoic lower crust
6.6 Speculation on the high electrical conductivity in the lower crust
7 Water in the deep continental lithosphere beneath the North China Craton
7.1 Preservation of initial water content in mantle minerals
7.2 Estimation of the ascent rate of xenolith/hosted alkaline basalts
7.3 Lateral variation of water content in the continental lithosphere
7.4 Vertical variation of water content between the lower crust and upper mantle
7.5 Implications on the rheological viscosity of the deep continental lithosphere
8 REE distribution and H-O-Li stable isotopes of lower crustal granulite minerals
8.1 Partitioning of REE between coexisting phases
8.2 Oxygen isotopic compositions
8.2.1 Isotopic vs. cation exchange temperatures
8.2.2 Recycled crustal materials during the petrogenesis
8.2.3 Inter-grain δ18O heterogeneities
8.3 Hydrogen isotopic compositions
8.3.1 Fractionation of H-isotopes between cpx and plag
8.3.2 Possible origins for the δD variations
8.3.3 Constraints on fluids in the continental lower crust
8.4 Lithium contents and isotopic compositions
8.4.1 Li-abundance and isotopic systematics
8.4.2 Partitioning and isotopic fractionation of Li between lower crustal minerals
8.4.3 Possible origins for δ7Li in the granulite minerals
9 Conclusions and future work


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