The Precambrian evolution of the Tarim Craton in NE China, in particular during the early Precambrian stage, remains enigmatic. In this contribution, we report field observation, petrology, geochemistry, zircon Lu-Hf isotopes and U-Pb ages of the major rock formations of the Aketage area in the southeastern section of the Tarim Craton. The Milan Group in Aketage is dominantly composed of 2.7-2.5 Ga gneissic amphibolite–TTG complex with minor paragneiss. Both the mafic and silicic rocks exhibit geochemical features consistent with an arc affinity. The arc-signature of the 2.01-2.03 Ga gneissic granites and gabbros which intrude the Archean basement, as well as the major 2.0 Ga metamorphic event revealed by zircon U-Pb dating, suggest an important subduction-collision event possibly related to the assembly of the Paleoproterozoic Columbia supercontinent. The ca.1848-1856 Ma massive potassic granites, 1867 Ma mafic dyke swarm and 1844 Ma massive leucogranite dykes reveal magmatism in a post-collisional extensional setting.
A comprehensive synthesis of the major orogenic events and continental crust growth process from the different Precambrian terranes in Tarim Craton show significant discrepancy in time related to the late Neoarchean crust formation ages and the Paleoproterozoic orogenic events. For example, the major orogenic event took place at ∼1.85 Ga in Quruqtagh-Dunhuang terrane, at ∼1.90 Ga in the southwest Tarim Craton and at ∼2.0 Ga in the Aketage-Qaidam terrane. These different terranes exhibit distinct periods of continental crust growth in the early Precambrian. Continental growth in the Aketage area took place during 2.7 Ga to 4.3 Ga. The 3.6 Ga xenocrystic zircons as well as the peak of 4.2 Ga zircon Hf model ages, indicate the possible existence of Paleoarchean and even Hadean crust in the Aketage area. In the Quruqtagh-Dunhuang terrane, the growth of early Precambrian continental crust took place at 2.6-3.3 Ga with peaks at ca. 2.6-2.7 Ga and 3.0 Ga. The diachronous late Paleoproterozoic orogenic events and the significant difference in continental growth process suggest that the Precambrian basement of the Tarim Craton is composed by independent continental terranes possibly detached from the cores of discrete ancient cratonic nuclei, which were not unified until the early Neoproterozoic during assembly of the Rodinia supercontinent.
The northern margin of the Tarim Craton plays a pivotal role in understanding the crustal evolution and supercontinent reconstruction of the Tarim Craton. Here we integrate LA-ICP-MS U–Pb ages and Hf isotopic data for detrital zircons from Neoproterozoic successions in the Aksu area, NW Tarim. A total of 679 concordant U–Pb ages define four age populations of 2600–2200 Ma, 2050–1800 Ma, 950–700 Ma and 680–600 Ma, which are consistent with the episodes of magmatism and metamorphism documented in the northern Tarim Craton, suggesting that the detritus were likely sourced from the northern Tarim Craton itself. The oldest age population corresponds to the Neoarchean to early Paleoproterozoic magmatic event related to an important stage in the development of the proto continental crust in the Tarim Craton. The 2050–1800 Ma age population represents a magmatic–metamorphic event during a middle Paleoproterozoic orogeny, possibly related to the assembly of Tarim to the Columbia supercontinent. The dominant Neoproterozoic detrital zircons display a major cluster between 950 Ma and 700 Ma (peak at ca. 850 Ma), much younger than the typical Grenvillian ages. This is similar to detrital zircon data sets from Yangzte and northern India, implying that these blocks shared a similar evolution pattern in the Rodinia supercontinent. The early Pan-African (680–600 Ma) ages are comparable with those reported from the Arabian-Nubian Shield, but quite different from those from other Gondwana blocks, implying a possible correlation of Tarim with the Eastern Africa Orogen. Abundant zircons yield Archean (3.9–2.5 Ga) Hf model ages (TDMC), suggesting the presence of extensive Archean basement as old as Paleoarchean in the Tarim Craton. Some Neoarchean and Neoproterozoic zircons have high positive ɛHf(t) values, indicating significant juvenile addition from the depleted mantle at those periods. However, most detrital zircons show negative ɛHf(t) values, suggesting that crustal reworking was the dominant process in the generation of the episodes of magmatism in the Tarim Craton. At last, based on the stratigraphic settings and the comparison of detrital zircon data, the Aksu Group and the Qiaoenbrak Formation are most likely units with equivalent depositional age.
Extensive granitoid and mafic–ultramafic magmatism and crustal growth occurred at ∼2.7 Ga in many cratons of the world, but the geodynamic setting during this period is complicated because both arc- and plume-related metavolcanic rocks are closely associated in several greenstone belts. Here, we present in situ zircon U–Pb–Hf–O isotopic and whole-rock geochemical data for meta-igneous rocks from the Korla Complex, northern Tarim Craton, NW China. SHRIMP and/or LA-ICP-MS zircon U–Pb dating indicates that two orthogneisses and an amphibolite crystallized at ∼2.71–2.74 Ga and were affected by at least two metamorphic events at ∼2.0–1.8 Ga and 0.8–0.6 Ga, respectively. These are the oldest rocks with reliable crystallization ages so far identified in the northern Tarim Craton. These rocks exhibit heterogeneous zircon Hf isotopic compositions, with the most radiogenic analyses (average ɛHf(t) = +7.4, n = 8) plotting on the depleted mantle evolution array and the most unradiogenic ɛHf(t) extending down to −5.6. This implies that both ∼2.7 Ga depleted mantle and ancient continental crust at least as old as 3.4–3.5 Ga contributed to the magma source. This conclusion is supported by zircon O isotopic data and Hf–O isotopic modeling of the orthogneisses. Available zircon U–Pb ages and Hf isotopic data show that two important Neoarchean magmatic events occurred at ∼2.71 and ∼2.55 Ga in northern Tarim, and that both events involved synchronous crustal growth and reworking. This observation suggests that the apparent peaks of zircon Hf crustal model ages of these rocks do not represent the time of crustal growth but are artifacts of magma mixing. Geochemical data show that the parent mafic magmas of the amphibolites follow two distinct evolution trends: a Fenner trend of extreme Fe–Ti enrichment (Group I) and a typical tholeiitic trend with Fe–Ti enrichment followed by Fe–Ti depletion (Group II). This was probably controlled by the fractionation of Fe–Ti oxides, which was in turn controlled by magma oxygen fugacity. The orthogneisses follow the second trend and can be interpreted as the products of assimilation and fractional crystallization of the Group II magmas. Trace element systematics indicate that the parent magma of Group I amphibolites resembled Nb-enriched or high-Nb arc basalts, whereas the Group II amphibolites were probably derived from tholeiitic arc basalts. Such a rock assemblage is similar to the island arc volcanic association found in ∼2.7 Ga greenstone belts in other cratons and indicates subduction of young and hot oceanic crust under an ancient continental block, which was probably an important process in the growth and differentiation of continental crust in the Archean.
Continental crust was largely generated before 2.5 Ga through mafic-ultramafic and TTG (tonalite-trondhjemite-granodiorite) magmatism, but it is contentious when did such primitive crust evolve into mature granodioritic to granitic composition similar to modern upper crust. Here we present zircon U-Pb-Lu-Hf-O isotopic data for late Paleoproterozoic metasedimentary rocks (Xingditag Group) in the Kuruktag area, northern Tarim Craton, NW China. CL-imaging reveals core-rim structures for most zircons from a garnet-bearing paragneiss and a semi-pelitic schist, whereas two quartzites are dominated by metamorphic zircons. SHRIMP and/or LA-ICP-MS U-Pb dating yielded a range of detrital ages from ca. 2.0 – 3.5 Ga for the zircon cores and a consistent metamorphic age of ca. 1.93 Ga for the rims for the paragneiss and schist. However, zircons from the two quartzites mainly record a ca. 1.85 Ga metamorphic event; detrital zircons are rare or absent. These data confirm that the Xingditag Group was deposited after ca. 2.0 Ga and was metamorphosed at ca. 1.93 and/or 1.85 Ga. Importantly, the ca. 2.0 – 3.5 Ga concordant detrital zircons exhibit low initial 176Hf/177Hf ratios (as low as 0.28045) and high δ18O values (6.6 – 11.4‰). These values are interpreted as recording primary magmatic features of the basement rocks in the northern Tarim Craton, because: 1) the dominantly prismatic or fragmentary morphology, oscillatory zoning and moderate Th/U ratios of the detrital zircons indicate a local provenance dominated by igneous rocks; and 2) the within-grain and overall heterogeneities argue against Hf and O isotopic resetting during metamorphism. Linear regressions of the initial 176Hf/177Hf values of these detrital zircons yield a remarkably consistent 176Lu/177Hf ratio of 0.01 for the oldest (TDM2 = 3.9 and 3.7 Ga) and youngest (TDM2 = 2.8 Ga) crustal components. These observations suggest that significant amounts of felsic continental crust may have been formed, altered and reworked as early as ca. 3.5 Ga, marking crustal differentiation and maturation during the Paleoarchean. Hafnium crustal model ages reveal that the oldest crustal component in the northern Tarim Craton may have been generated before ca. 3.9 Ga, much earlier than previously thought.
The Quanji Massif, located in the northeastern margin of the Tibet Plateau, is interpreted as a fragment of the Tarim Craton. The massif exposes a suite of metamorphosed mafic dykes. LA-ICP-MS U–Pb dating constrains the timing of intrusion of these dykes at ca. 1834 ± 23 Ma. The dykes display minor variations in major elements, with SiO2 = 46.8–53.4 wt%, MgO = 5.19–8.10 wt%, FeOt = 8.83–15.6 wt%, TiO2 = 0.58–1.78 wt% and Mg# = 46.3–69.7. Their immobile trace element compositions show a sub-alkali basalt affinity. The positive correlation of TiO2 with FeOt/MgO in these rocks shows an arc tholeiite evolutionary trend. The enrichment of LILE and LREE and depletion in HFSE suggest that the precursor magma was generated in a back-arc environment. Furthermore, these rocks possess (Nb/La)N of 0.31–0.52 and (Th/La)N of 0.61–1.39, whole rock (87Sr/86Sr)i values of 0.707598–0.724141, ɛNd(t) of −2.9 to +0.3 and (206Pb/204Pb)t of 16.8148–24.1513, (207Pb/204Pb)t of 15.3422–16.3630 and (208Pb/204Pb)t of 36.8415–39.7926. The magmatic zircons yielded ɛHf(t) of −3.4 to +6.1 and depleted mantle model ages (TDM) of 2.01–2.37 Ga. The geochemical and isotopic characteristics of these rocks suggest that their precursor magma was derived from a subduction-related fluid-metasomatized subcontinental lithospheric mantle mixed with a depleted mantle component. We suggest that a prolonged subduction-accretion-collision process along the southeastern margin of the Tarim Craton prevailed at ∼2.1–1.80 Ga, with local consumption of the Southeast Tarim Archipelagic Ocean during the collision of the Southeast Tarim Block with the Quanji Massif (microcontinent) at around 2.1–1.9 Ga. The final closure of the whole Southeast Tarim Archipelagic Ocean occurred at ∼1.85–1.82 Ga and collision between the Southeast margin of the Tarim Craton and other unknown continental blocks at 1.82–1.80 Ga, broadly coeval with the amalgamation of the Western and Eastern Blocks of the North China Craton and their assembly in the Columbia supercontinent.
Metadiabase sills are extensively distributed in the Astingbulake region in the central Quruqtagh block of the NE Tarim Craton. Here we report laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb ages and Hf isotopes, as well as whole rock elemental and Sr-Nd isotopic data for these Mesoproterozoic sills. Zircons from one of the mafic sills yield an emplacement age of 1470 ± 9 Ma (95% confidence, MSWD = 3.2, n = 23) Ma. Despite the greenschist-amphibolite facies metamorphism, most of the immobile elements provide important clues for the petrogenesis and tectonic settings of these rocks. Except for their variable fluid mobile-element contents, all the studied samples show enrichment in incompatible trace elements with no obvious Nb–Ta depletion, similar to the features of continental flood basalts and ocean island basalts (OIB). Combined with their relative higher 87Sr/86Sr (t) ratios (0.70666 to 0.70784), negative ɛNd(t) (-4.30 to -3.96) and ɛHf (t) (-5.49 to -1.13) values, we propose that the protolithic magmas were derived from the high degree partial melting of enriched continental lithospheric mantle within continental rifting settings. The ∼1.5 Ga diabase sills from North Tarim corresponded to the major episode of mafic magmatism during Early Mesoproterozoic time identified from other crustal fragments of Laurentia, Siberia, Greater Congo and South China, and probably belong to one of the three major large igneous provinces associated with the breakup of the Mesoproterozoic Columbia supercontinent. Our data provide important constraints on the configuration of the Tarim Craton within the Columbia supercontinent
