1.1 Tectonic Evolution of the Tarim Block

The Tarim Block and the North and South China Blocks make up the three major continental blocks in China. The Tarim Block occurs within the Xinjiang Uygur Autonomous Region of northwestern China and covers an area of more than 6×105 km2. It is surrounded by the Tianshan orogen to the north, the Kunlun orogen to the south, and the Altyn-Tagh orogen to the southeast (Fig. 1.1). The main part of the Tarim Block is the Tarim Basin. The Tarim Basin can be divided into several tectonic units including the Kuqa Depression, the North Uplift, the North Depression, the Central Uplift, the Southwest Depression, the Southeast Uplift and the Southeast Depression (Fig. 1.1 and Fig. 1.2). A series of important tectonic movements occurred within superimposed basins during different periods. The features and textures of proto-type basins were generally superimposed and reconstructed by tectonic movements in later periods due to the unstable tectonics of the Tarim Block which resulted from the relatively smaller scale of the Tarim Craton Block and multiple, intense episodes of tectonic movements in peripheral areas (Jia and Wei, 2002; Jia et al., 2004).

The Tarim Block is a cratonic block with Archean and Paleo- to Meso-Proterozoic crystalline basements. Sedimentary cover is composed of Neo-Proterozoic, Paleozoic, Mesozoic and Cenozoic.

Most of the Tarim Block is occupied by desert, but outcrops of Precambrian and Paleozoic to Cenozoic rocks are scattered along its margins. The Tarim Block is characterized by a double-layer structure consisting of a metamorphic basement overlain by the Late Neoproterozoic to Cenozoic sedimentary cover sequences.

1.1.1 Evolution of the Basement

The Tarim Block has experienced several stages of tectonic evolution since its formation, and the previous geochronological data from the TTG (Trondhjemite-Tonalite-Granodiorite) and other rocks from the Quruqtagh, Altyn-Tagh and Tiekelike areas suggest that the Tarim Block was mainly built up in several tectonic episodes at 2.65-2.45 Ga, 2.0-1.8 Ga and 1.1-0.9 Ga (Hu et al., 2000; Lu and Yuan, 2003; Zhu et al., 2008; Lu et al., 2008).

The metamorphic basement of the Tarim Block is mainly composed of Archean to Early Neoproterozoic metamorphosed strata and magmatic rocks and it mainly crops out in four areas surrounding the orogenic belts of the basin, i.e. the Korla—Quruqtagh area, the Aksu—Keping area, the Tiekelike area and the Altun—Dunhuang areas at the NE, NW, SW and SE margins of the Tarim Block, respectively (Fig. 1.3). The basement consists of Neoarchean TTG gneisses with minor supracrustals, Paleoproterozoic mafic-felsic intrusions, high-grade supracrustals and minor anatectic granites, and Late Mesoproterozoic to Early—Middle Neoproterozoic meta-sedimentary and volcanic strata metamorphosed in greenschist and blueschist facies, which are together unconformably overlain by Late Neoproterozoic Sinian unmetamorphosed cover. This formation and evolution were closely related to the assembly and breakup of the supercontinents of Columbia (Nuna) and Rodinia (Lu et al., 2008; Zhang et al., 2012).

Neoarchean to Paleoproterozoic rocks in the Tarim Block mostly outcrop along its eastern and northern margins which are mainly exposed in the Quruqtagh and Dunhuang complexes, and include the Neoarchean tonalitic granitic rocks and the Paleoproterozoic amphibolite to granulite facies paragneiss, most of which were emplaced in the period 2.60-2.50 Ga (Lu, 1992; Long et al., 2010, 2011; Shu et al., 2011; Zhao and Gawood, 2012; Zhang et al., 2012). In most places, the Archean rocks outcrop as stripes or lenses with variable dimensions that are tectonically enclosed within the Paleoproterozoic paragneiss; both of them generally show foliations that are concurrent to each other (Zhang et al., 2012). These features suggest that the Archean and Paleoproterozoic rocks had undergone the same tectono—metamorphic event in the Paleoproterozoic Era because the low-grade metamorphic Mesoproterozoic unconformably overlies on the Archean and the Paleoproterozoic rocks (Xijiang BGMR, 1993). In the Qulukatage Complex, these Neoarchean and Paleoproterozoic rocks underwent two metamorphic events at 1.9-1.8 Ga and 1.1-1.0 Ga, which are considered as having been related to the assembly of the Columbia and Rodinia supercontinents, respectively (Shu et al., 2011; Zhang et al., 2012).

Late Mesoproterozoic to Early—Middle Neoproterozoic metamorphosed strata are exposed on the peripheral margins of the Tarim Block, represented by the Bowamu, Aierjigan and Aksu groups on the northern margin, the Kalakashi (Sailajiajitage) and Ailiankate groups on the southern margin, the Bulunkule Group on the southwestern margin, and the Altyn-Tagh Group on the southeastern margin. Most of these groups are considered to have formed in Andean-type continental margins, which were deformed and metamorphosed at 1.0—0.9 Ga, probably related to the assembly of Rodinia (Zhang et al., 2003; Lu et al., 2008).

1.1.2 Evolution of Sedimentary Cover Sequences

Since the late Middle Neoproterozoic, the Tarim Block has become a stable platform overlain by late Middle Neoproterozoic to Cambrian unmetamorphosed cover sequences (Fig. 1.4). The late Middle to Late Neoproterozoic sequences are called the Nahua and Sinian System containing four sequences of tillite, interpreted as evidence for the Neoproterozoic Snow Ball Earth Event. During the Nahua to Sinian, the Tarim Block began to break up during separation from the Rodinia supercontinent. Rifting-related mafic igneous rocks are widely distributed both in the northern and southern margins of the Tarim Block. The Nahua and Sinian sequences were deposited on the Pre-Nahua crystallized basement unconformably and are composed of glacial deposit conglomerate and terrigenous clastic deposits. The Sinian, overlying unconformably on the Nahua, consists mostly of dolomite and mudstone intercalated with basalt. The thickest deposits of the Sinian in the basin are found in the Manjiaer depression, which was controlled by a group of faults and filled with about 1000 m of deep marine mudstone and muddy limestone with rift volcanic rocks.

The Paleozoic series in Tarim exhibit typical features of the sedimentation formed at passive continental margins (Fig. 1.4). Along the southern—southeastern side and the northern side, the thickness of the Paleozoic reaches up to 12 km, and in central Tarim the thickness varies from 5000 m to 8000 m according to data from the boreholes (Jia et al., 2004). The thickness of the Paleozoic sequences in Tarim was strictly constrained by the sedimentary troughs between the Tarim and the neighboring orogens, i.e., the Tianshan orogen in the north and the western Kunlun—Altyn-Tagh orogen in the south and southeast. The large-scale transgression took place during the Early Cambrian and deposited a set of deep-water dark mudstones with phosphates at the base of the Cambrian over most parts of the block, which comprises one of the most important hydrocarbon source rocks in the basin. These pass upward into thin-bedded dolomites, thick gypsum and salt layers intercalated with dolomites (from the Middle Cambrian), formed in an evaporative carbonate platform and slope environment. The Upper Cambrian is made up of thick-bedded dolostone, and the Lower to Middle Ordovician consists mainly of thick dolomitic limestones and the low part of the Upper Ordovician is mainly made up of carbonate deposits formed in open platform environments with reef and shoal deposits along the platform margins. These deposits are overlain by silicic clastic abyssal deposits of the upper part of the Upper Ordovician.

From the Silurian to the Lower—Middle Devonian contact with Pre-Silurian was a widespread angular unconformity caused by collision between Tarim Block and North Kunlun Block (Fig. 1.4). This tectonic event also ended the development of the Late Ordovician deepwater basin. The Silurian basin is filled with fluvial, deltaic, or clastic littoral deposits, and the Lower to Middle Devonian is composed of fluvial and littoral deposits with red beds. At the end of the Middle Devonian, the Tarim Block suffered strong deformation and denudation with the largest erosion amount of up to 3000-5000 m along the northern and northeastern margin, forming an angular unconformity distributed over most parts of the Tarim Block.

During the Late Devonian, the tectonic geomorphology was generally high in the northeast and low in the southwest from the Late Devonian to the Carboniferous. This significantly constrained the distribution of the Late Devonian to the Early Carboniferous paleogeography of the block. As transgression took place northeastwards, the Donghetang Formation was composited by shoreline and wave-dominated deltaic deposits predominated by clean quartz sandstones. During the Early Permian, one notable feature of the Tarim Block was the wide occurrence of the Early Permian intra-plate magmatism in which the magmatic rocks were made up mainly of basaltic rocks, including basalts, diabase, basaltic andesite, ultramafic rocks, etc. The residual area of the magmatic rocks can reach a size of about 2.5×105 m2, and this magmatism resulted in a regional unconformity and onlap contact between the earliest Permian and Carboniferous strata with denudation of the Carboniferous and the deposition of the earliest Permian strata. During the Permian, the deposition of the Tarim Block evolved from a marine into a fluvial and lacustrine intercontinental basin, which caused the uplift and paleogeographic change at the beginning of the Permian.

The Triassic is made up of terrigenous clastic deposits, including alluvial and lacustrine deposits (Ji et al., 2003), which can be classified into three subordinate tectonostratigraphic units or sequences (Fig. 1.5). There was an inland depression in the central basin as well as the Kuqa foreland depression along the northwestern margin of the basin; the formation of the latter was related genetically to the collision of the South Tianshan orogeny and the Tarim Block during the Triassic (Zhang et al., 2007a, b). In the Kuqa depression, the Triassic deposits formed a wedge that thickens toward the northern marginal foredeep.

Jurassic sediments deposited on pre-Jurassic ones with a lack of unconformity, which is a block scale of unconformity, and heavy erosion, can be recognized and traced on seismic profiles in most parts of the block, particularly in the northeastern part of it. The nature of the Jurassic Tarim Block was in regional extension although there has long been a debate on it (Sobel, 1999; Wu et al., 2005). The Lower Jurassic Ahe Formation is composed of alluvial fan and braided fluvial conglomerates and sandstone. The overlying Yangxia Formation of the Lower Jurassic is dominated by shallow and deep muddy lacustrine deposits. There is an unconformity at the base of the Middle Jurassic, and the fluvial sandstones of the Kezhilenuer Formation of the Middle Jurassic irregularly overlying the thick mudstones of lacustrine deposits of the Yangxia Formation in most parts of the block. The Lower Cretaceous includes alluvial, fan delta or braider river delta, shoreline, and shallow or semi-deep lacustrine and arid salt lake deposits. The Upper Cretaceous in the Southwest Depression contains thin beds of fossiliferous mudstones, bioclastic limestone, reef limestone, and gypsum mudstone of marine and lagoon, tidal deposits, which are associated with shoreline clastic and deltaic deposits.

The Eogene and Neogene can be subdivided into four composite sequences in the Kuqa depression, and each of them ranging from 400 to 1000 m in thickness, are confined by unconformities and are made up of regional depositional cycles evolving from alluvial or fluvial to lacustrine and finally to fluvial deposits. The basal surface of the Kumugelimu Formation is a widespread minor angular unconformity, and the lower part of the Kumugelimu Formation has thick beds of alluvial fan deposit overlying the Lower Cretaceous. It has been suggested that the formation of these sequences is controlled by the foreland tectonic process from flexural subsidence caused by thrust loading to rebounded uplift due to the erosion and stress release (Fig. 1.2). In the central foredeeps, the upper part of the Eogene contains thick beds of gypsum mudstones and salt beds and some thin beds of limestones deposited in a lagoon or embayment environment related to instantaneous marine transgression. Up until the Miocene, the compression from orogenic belts intermittently increased and the foredeeps along the foreland of the Tianshan and Kunlun Mountains were thrusted, leading to a series of faulted and folded structural belts.

1.1.3 Igneous Activities

Major episodes of the igneous-metamorphic activities in the Tarim Block include those during the Late Archean, the Paleoproterozoic, the Late Mesoproterozoic to the Late Neoproterozoic, and the Early Permian.

The Late Archean include the tonalitic granitic rocks (as well as some gabbro enclaves in granitic rocks). In most places, the Archean rocks outcrop as stripes or lenses with variable dimensions that are tectonically enclosed within the Paleoproterozoic paragneiss; both of them generally show foliations that are concurrent to each other (Zhang et al., 2012). The Late Archean gneissic granites in the Quruqtagh area, as well as some gabbro enclaves in the granites, were termed as the Tuogelakebulake complex. Petrographically, the Late Archean rocks could be divided into three groups, i.e., the TTG (tonalite, trondhjemite, granodiorite as well as granites), calc-alkaline granites and high Ba—Sr granites.

The Early Paleoproterozoic igneous activity was documented in the southwestern, northern and eastern parts of the Tarim Block. The rock types include A—S granites with gabbro enclaves on the southwestern margin, calc-alkaline granites in the Quruqtagh area, gneiss granites and mafic dikes in the eastern Tarim Block (Zhang et al., 2007b; Lu et al., 2008; Long et al., 2010). According to geochemical studies, these igneous rocks were formed in an extensional environment (Zhang et al., 2007b; Lu et al., 2008). Lei et al. (2012) reported 1944—1934 Ma granites in the Xishankou area near Korla City, and these granites show continental-arc-type geochemical signatures.

The gneissic granites from the Late Mesoproterozoic period to the Neoproterozoic mostly outcrop in the Quruqtagh and Altyn—Dunhuang areas (Deng et al., 2008; Lu et al., 2008; Shu et al., 2011) and ca. 1.0 Ga diorite of arc signatures was reported in the central—Tarim area (Li et al., 2005). In combination with the coeval metamorphism (1.0—0.9 Ga) around the Tarim Block, it was suggested that this phase of igneous-metamorphic activity was intimately related to the assembly of the Rodinia supercontinent. Neoproterozoic igneous activities along the northern margin of the Tarim Block could be broadly divided into four phases. They are as follows: (1) the ca. 820—800 Ma ultramafic—mafic—carbonatite complex and voluminous adakitic granites and mafic dike swarm; (2) the ca. 780—760 Ma tholeiitic ultramafic—mafic complex and voluminous mafic dike swarm; (3) the ca. 740—735 Ma bimodal complex and bimodal volcanic series; and (4) the ca. 650—635 Ma mafic dikes (Chen et al., 2004; Zhan et al., 2007; Zhu et al., 2008; Zhang HA et al., 2009; Zhang CL, 2010a, 2012). The presence of voluminous mafic rocks with an arc-like signature and the calc-alkaline granites in this area suggest that there was a partial melting of the enriched lithospheric mantle and crust in response to the Rodinian breakup (Zhang et al., 2013). The formation of enriched sources may result from the interaction between the overlying lithosphere and the oceanic crust due to subduction beneath the Tarim continent during the Grenvillian orogeny (Zhang C.-L. et al., 2011, 2012, 2013).

The Early Permian magmatic rocks display a great variety in lithology, ranging from ultramafic, mafic to felsic compositions and all of them exhibit typical within-plate affinities in geochemistry.