Geological background and previous works

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Geological background and previous works

The continental regions on the Earth consist of several Precambrian cratons and Phanerozoic orogenic belts between them (Fig. 2-1). These cratons, with either Archean or Precambrian basements, represent the cores of continents that have not been deformed for at least 1 Ga, in strong contrast to the active orogenic belts (Lee et al., 2011).
The North China Craton (NCC) is relatively small among those cratons with Archean basements worldwide. From a view of a smaller scale, The NCC is one of the main tectonic unites in the eastern China, which comprises other three tectonic unites: from north to south, the eastern Central Asian Orogenic Belt, the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, and the South China Craton (Fig. 2-2). These units were amalgamated during the Phanerozoic. Inside the NCC, it can be subdivided into three parts: the eastern NCC, the western NCC and the central NCC between them (Zhao et al., 2000, 2001). These tectonic units in the eastern China are detailed below.

The North China Craton

The North China Craton (NCC) is the Chinese part of the Sino-Korean Craton (SKC), and is also can be seen as the North China Block (NCB) in the literature. It is one of the most ancient cratons on the Earth, composed of early Archean and Proterozoic basements with the oldest recorded crustal ages >3.8 Ga (e.g. Liu et al., 1992), and is also the largest craton in China, covering an area >1,700,000 km. It is bounded by the Central China orogeny (including the Qinling–Dabie Shan–Sulu belts) to the SW, and the Inner Monglia–Daxinganling orogenic belt (the Chinese part of the Central Asian Orogenic Belt) on the north (Figure2-1 and 2-2). The western boundary is more complex, where the Qilian Shan and Western Ordos thrust belts obscure any original continuity between the NCC and the Tarim Block. The location of the southeastern margin of the craton is currently under dispute (Oh and Kusky et al., 2007), with uncertain correlations between the North and South China Cratons and different parts of the Korean Peninsula. The Yanshan belt is an intracontinental orogen that strikes east–west through the northern part of the craton (Davis et al., 1994; Bai and Dai 1998).
There are two major large-scale geological and geophysical linear zones cutting across NCC (Fig. 2-2). To the west, the craton is cut by the Daxing’anling-Taihangshan Gravity Lineament (DTGL); to the east, the craton is traversed by the Tan-Lu Fault Zone (TLFZ). Based on the lithological assemblage, tectonic evolution and P-T-t paths of metamorphic rocks, the North China Craton can be divided into the Western and Eastern Blocks, separated by the Central Orogenic Belt, the assembly of the two blocks during the Proterozoic cratonization (Zhao et al., 2000; 2001). The Western Block is composed of late Archean to early Proterozoic metasedimentary belts that unconformably overlie the Archean basement; the latter consists mainly of granulite facies gneiss and charnockite with small amounts of mafic granulites and amphibolites. The basement of the Central Orogenic Belt consists of late Archean amphibolites and granulites, and 2.5 Ga granite-greenstone terrains with overlying 2.4-2.2 Ga bimodal volcanic rocks in the southern region and thick carbonate and terrigenous sedimentary rocks interleaved with thin basalt flows in the central region. The Eastern Block is composed of late Archean orthogneisses intruded by 2.5 Ga syntectonic granitoids. The collision between the Western and Eastern blocks 1.8-2.0 Ga ago may have led to the formation of the Central Orogenic Belt and the final amalgamation of the North China Craton.
In detail, The NCC includes several micro-blocks and these micro-blocks amalgamated to form a craton or cratons at or before 2.5 Ga (Geng 1998; Kusky et al., 2001, 2004, 2006; Kusky and Li, 2003; Zhai 2004; Zhao et al., 2000, 2001), although others have suggested that the main amalgamation of the blocks did not occur until 1.8 Ga (Zhao et al., 2001, 2006; Liu, S.W., et al., 2004, 2006; Guo et al., 2005; Kroner et al., 2005a, b, 2006). Exposed rock types and their distribution in these micro-blocks vary considerably from block to block. All rocks .2.5 Ga in the blocks, without exception, underwent the 2.5 Ga metamorphism, and were intruded by 2.5– 2.45 Ga granitic sills and related bodies. NdTDM models show that the main crustal formation ages in the NCC are between 2.9 and 2.7 Ga (Chen and Jahn, 1998; Wu et al., 2003a, b). Emplacement of mafic dyke swarms at 2.5–2.45 Ga has also been recognized throughout the NCC (Liu, 1989; Li, J.H. et al., 1996; Li, T. S. 1999).
The NCC, however, did not survive the way by which most of its counterparts worldwide did, such as the Kaapvaal Craton (South Africa), the Slave craton (North Canada) and the Siberian Craton. It experienced widespread tectonothermal reactivation since the Phanerozoic (Menzies et al., 1993; Xu, X.S. et al., 1998, 2004; Menzies et al., 2007 and the references therein). Its reactivation was well recorded by lines of evidences. First, the growing activities of magmatism in the NCC is the most straight evidence, beginning from the Ordovician with the intrusions of kimberlitic magmas through the thickest part of the craton, and culminating in the late Mesozoic-Cenozoic with voluminous intrusive and volcanic rocks of varying compositions, ranging from mafic to dominantly felsic (Menzies et al., 2007 and the references therein). Some even argued that the reactivation could begin as early as Proterozoic ages, on the ground that the dominance of Proterozoic Re-Os model ages over the Archean ones within the mantle peridotite suites may imply that significant changes occurred to the NCC, with the archean roots replaced by the younger material during the Proterozoic (Gao et al., 2002; Wu et al., 2003b). Second, 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).
These changes in both thermal state and chemical composition of the lithospheric mantle were best recorded and constrained by mantle xenoliths of rocks and mineral concentrates. Xenoliths carried in Ordovician kimberlitic magmags are deep-seated garnet-facies peridotites. These xenoliths, together with the appearance of diamonds in the kimberlites and the P-T equilibrium conditions preserved in the inclusions/mineral concentrates of these diamonds, indicate a “shield” geotherm for the Palaeozoic lithosphere, characterized by low heat flow of 40 mW/m2, and thick lithospheric keel (~ 200 km). By contrast, xenoliths hosted in late Cretaceous and Cenozoic basalts are dominated by fertile spinel-facies peridotites, which represent a shallower and hotter lithospheric mantle from the thermobarometry, in good agreement with an average present-day surface mean heat flow of 80 mW/ m2 and thin lithosphere of 60-80 km from the geophysical observations. And it was also a compositionally heterogeneous lithospheric mantle (Fan and Hooper, 1989; Xu et al., 1995; Xu, X.S. et al., 1998; Zheng et al., 1998, 2001, 2006; Fan et al., 2000; Rudnick et al., 2004; Reisberg et al., 2005; Ying et al., 2006). Collectively, these lines unambiguously demonstrate that more than 100 km of the Archean root was removed or strongly modified during late Mesozoic-early Cenozoic time beneath the NCC, at least the Eastern part (Menzies et al., 1993; Griffin et al., 1998; Xu, 2001, 2007; Zheng et al., 2001, 2006; Gao et al., 2002, 2004, 2008; Zhang et al., 2002, 2005, 2008, 2009a; Wu et al., 2006). The mechanism and tectonic driving force responsible for the NCC lithospheric thinning have been intensely debated during the last decade (Menzies et al., 2007 and references therein).

Lithospheric thinning and the destruction of the NCC

The lithospheric thinning in eastern China was observed as early as 1990s, and described in different works. (Fan and Menzies, 1992; Menzies et al., 1993; Griffin et al., 1998; Menzies and Xu, 1998; Menzies et al., 2007), but was first formally identified as a scientific issue by Menzies et al. (1993). It did not catch too much attention until recently (mostly, the past decade) when it was linked to the destruction of the craton. Although there is no intrinsic connection between the two, the relation is indeed obvious in many ways. It also remains unclear, however, over the cause, extents, mechanism and timing, as well as tectonic controlling factors, for this geodynamical process.
The scale for the lithospheric thinning involves both its horizontal and vertical distribution, which indicates how wide and how thick the lithosphere has been removed, respectively. There is at present a general consensus that the NCC has experienced significant lithospheric thinning, especially its eastern domain. Recent studies show that, not only the NCC, but also the north-east and south-east China are characterized by rather thin lithosphere relative to other ancient cratons on the Earth (Zou, 2001; Xu et al., 2002; Wu et al., 2003a). Based on these work, it seems that the entire eastern China, to the east of the DTGL, has experienced lithospheric thinning. This can be further supported by the thermal structure (Fig. 2-3a: He et al., 2001) and seismic tomography (Fig. 2-3b: Priestley et al., 2006) of the upper mantle beneath the eastern China. The issue over the vertical thinning involves the removal of either only the lithospheric mantle or both part of the lower crust and the underlying lithospheric mantle, which is still much debated until recently (Menzies et al., 1993; Griffin et al., 1998; Menzies and Xu, 1998; Zheng, 1999; Xu, 2001; Wu et al., 2003a, b; Gao et al., 2004; Xu and Bodinier, 2004; Menzies et al., 2007).
How the lithospheric keel below eastern China had been lost has been the subject of hot debate during the past 20 years, among which previous studies were largely focused on the eastern NCC, where the lithospheric thinning is most obvious. Several mechanisms or models have been proposed to explain this thinning below the NCC, which can be grouped into two end-members: the “top-down” rapid delamination models versus “bottom-up” protracted thermomechanical-chemical erosion models.
The Delamination model predicts a short period (less than 10 Ma) (Yang et al., 2003; Gao et al., 2004). Yang et al. (2003) suggested that lithospheric delamination took place primarily in the early Cretaceous, based on the evidence for widespread crustal melting during 130-110 Ma which would require thinning of the lithosphere. By contrast, Gao et al. (2004) argued for Jurassic delamination of the lower crust, based on their discovery that Jurassic andesites, dacites and adakites from Xinglonggou (north NCC) have chemical signatures consistent with their derivation as partial melts of eclogites that interacted with mantle peridotite; in this case, they proposed that the lithospheric thinning had reached such a stage by the late Jurassic that lower crustal rocks could be delaminated, converted to eclogites, incorporated into the convecting mantle and melted. The latter model, however, is difficult to reconcile with the fact that Mesozoic mafic and felsic magmatism peaked in the early Cretaceous (Yang et al., 2003; Xu et al., 2004; Wu et al., 2005) rather than the Jurassic; furthermore, rapid delamination is clearly at odds with the protracted Mesozoic magmatism (~100 Ma) in the NCC (Xu et al., 2004), and it is not easy to reconcile the linear thinning along the whole east China.
The thermomechanical-chemical erosion models, by contrast, hints at a protracted process, possibly up to 100 Ma (e.g. Griffin et al., 1998; Xu, 2001). In this scenario, lithospheric thinning proceeded by heat transport into the lithosphere and small-scale asthenospheric convection induced by extension. Once lithospheric mantle was thermally converted to asthenosphere, it can convectively mix with, and eventually be replaced by, the underlying asthenosphere (Davis, 1994). A recent hypothesis suggests that the lithospheric thinning has been initiated by hydration weakening, for which the water required may come from dehydration of the subducted Paleo-Pacific lithosphere that remained horizontally stagnant within the transition zone beneath eastern China (Niu, 2005). The westward thrust of the Pacific plate into the transition zone underneath east China has been recently observed by high-resolution seismic tomography (Huang and Zhao, 2006). Furthermore, the subduction may even lead to the gravity anomaly in eastern China and the formation of the DTGL in the early Cretaceous (e.g. Niu, 2005; Xu, 2007). This subduction-related mechanism is also supported by evidence from mineral compositions of mantle peridotites with different formation ages (Zheng et al., 2006). For the past two decades, the time-scale of such destruction has been debated, including its beginning, peak-period and ending. This, however, relies fundamentally on the well understanding of how the lithosphere keel has been removed. A short time interval of only 10 to 20 Ma, or even less, for delamination-induced thinning (Yang et al., 2003; Gao et al., 2004) is in strong contrast to that of over 100 Ma for erosion-induced thinning (Griffin et al., 1998; Xu, 2001; Xu et al., 2004). Some relevant geological, geochemical and geophysical data on a 200 Ma time scale for the NCC are summarized by Menzies et al. (2007), so that these events can be cross-correlated. The presence of mantle derived plutonic rocks around 180-190 Ma is believed to mark the reactivation of the cratonic lithosphere, the early Cretaceous is widely agreed to mark a key.
In addition, many other thinning models or mechanisms have been proposed, and are intermediate between the above two end member models. The differences in chemical composition and orgnization of the lithospheric mantle beneath the eastern North China Craton prior to and after its thinning according to different models are illuminated in Fig. 2-4
Figure 2-­‐4. Schematic diagram of the lithosphere beneath the eastern North China Craton prior to and after its thinning according to different models (From Zhang et al, 20009).

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The Dabie-Sulu UHP Belt

The Dabie-Sulu UHP belt, the eastern part of the Central China orogeny, lies between the North China Craton and South China Craton, extending from east to west for ca. 2000 km in the central-eastern China (Fig. 2-2). It is separated into two terrains by about 500 km of left-lateral strike-slip displacement along the TLFZ. At the east, the Sulu terrain is segmented into a number of blocks by several NE-SW trending faults that sub-parallel to the TLFZ. At the west, the Dabie terrain is the major segment, separated into a series of continuous zones by several EW-trending faults of large scales. The formation of the Dabie-Sulu UHP belt mainly in the Triassic was caused by collision between the North China and Yangtze Cratons with peak metamorphism at ~ 245 Ma (Hacker et al., 1998), followed by a series of closing of ocean basins. 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 cpx, rutile and apatite, in eclogites (e.g. Okay et al., 1989; Wang et al., 1989; Xu, S.T. et al., 1992; Yui et al., 1995; Ye et al., 2000) demonstrates the subduction of the continental crust to mantle depths of about 200 km and the subsequent quick exhumation (see also a review by Zheng, Y.F. et al., 2003). The chronological evolution in terms of the subduction and exhumation of the Dabie-Sulu UHP Belt can be simplified as the following: the peak metamorphism are at ~ 240-245 Ma, with differential exhumation processes to crustal levels in different areas, e.g. eclogites-facies recrystallization at ~ 230 Ma in central Dabie and granulitization at ~ 220 Ma in north Dabie, prior to amphibolite- facies retrogression at ~ 200 Ma (Zheng, Y.F. et al., 2003; and references therein).

Localities and samples

The deep xenoliths, from the lower crust and lithospheric mantle, can serve as time capsules for unveiling temporal evolutions of the lithosphere directly. Fortunately, the mantle xenoliths are widely distributed in the north China, south China, and eastern China of our interest in this study (Fig. 2-2). Our research area is the eastern part of the NCC, where the signs of the lithospheric thinning are most obvious. Mantle peridotite xenoliths in this thesis were collected across most of the mantle xenolith outcrops in Eastern NCC, ranging from the eastern domains in Shandong province (including six localities: Penglai, Qixia, Qingdao, Daxizhuang, Changle, and Junan) where the lithospheric mantle have suffered most the thinning and it is generally believed that all the archean roots have been all removed, to Hebi of Henan province, near the Trans-North China Orogen, where the subcontinental mantle have been relatively less modified and there are solid proofs for the ancient remainders (Zheng et al., 2001, 2006). Among these xenoliths those from Junan, Qingdao and Daxizhuang were exhumed by Mesozoic alkali basalts, and the others in Cenozoic basalts. Unfortunately, there is no mantle xenoliths hosted by the volcanic rocks that predated the late Cretaceous.

Table of contents :

1 Introduction
1.1 Background and motivation
1.2 Thesis outline
2 Geological background and previous works
2.1 The North China Craton
2.3 The Dabie-­‐Sulu UHP Belt
2.4 Localities and samples
2.4.3 Changle
4.3 FTIR results
4.3.1 H-­‐related species in mineral structural
4.3.2 Water contents in minerals of peridotite
4.3.2 Water contents in bulk peridotite
5. Conclusions

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