Precambrian Tectonothermal Events And Crustal Evolution In The Kuruktag Block, Northern Tarim Craton

The Kuruktag Block located in the northeastern Tarim Craton contains important Precambrian outcrops that record a long and complicated geological evolution history. Detailed cross-section measuring, systematic petrological observation, LA-ICP-MS and SHRIMP zircon U-Pb dating, in situ zircon Hf-O isotopical analysis, and geochemical analysis are carried out in this study for the Precambrian meta-plutonic and supracrustal rocks and the unmetamorphosed intruding granitoids in the Korla to Xishankou area in the western Kuruktag Block. Formation ages, petrogenesis and metamorphic and deformation history of these rocks were determined. Archean crustal evolution and geodynamic settings of Paleoproterozoic and Neoproterozoic tectonothermal events and their implications for the evolution of the Columbia and Rodinia supercontinents are discussed.Cross-section measuring indicates that a suite of metamorphic rocks mainly composed of meta-igneous rocks (orthogneiss, gneissic granitoids and amphibolite), metasedimentary rocks (mica schist, paragneiss, quartzite and marble) and migmatites is exposed in the Korla area and is termed the Korla Complex. SHRIMP and/or LA-ICP-MS zircon U-Pb dating indicates that two orthogneisses and an amphibolite in the Korla Complex crystallized at～2.71-2.74Ga and were affected by at least two metamorphic events at-2.0-1.8Ga and0.8-0.6Ga, respectively. These are the oldest rocks with reliable crystallization ages so far identified in the northern Tarim Craton. Zircon Hf-O isotopic features indicate that both～2.7Ga depleted mantle and ancient continental crust at least as old as3.4-3.5Ga contributed to the magma source of these rocks. Geochemical data show that the parent mafic magmas of the amphibolites resembled tholeiitic arc basalts and Nb-enriched or high-Nb arc basalts. The orthogneisses were probably the the products of assimilation and fractional crystallization of the parent magmas of some amphibolites. Such a rock assemblage is similar to the island arc volcanic association found in～2.7Ga greenstone belts in the Superior Province and the Dharwar Craton 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. Available zircon U-Pb ages and Hf isotopic data show that two important Neoarchean magmatic events occurred at～2.7and～2.5Ga in the northern Tarim Craton, 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.The Paleoproterozoic metasedimentary rocks in the Korla area, i.e., the Xingditag Group, experienced complicated metamorphism and migmatization. Mineral assemblage and P-T estimate for the mica schists indicate an upper amphibolite facies peak condition, with P=11±2kbar and T=690±50℃. These mica schist, quartzite and migmatitic paragneiss consistently record a～1.85Ga metamorphism. But most detrital zircons in these rocks might have been transformed into new metamorphic zircons through dissolution-reprecipitation. These newly grown metamorphic zircons exhibit relatively homogeneous initial176Hf/177Hf and may have resulted from mixing and isotopic homogenization of Zr-Hf released from dissolution of tiny detrital zircons and decomposition of garnet into chlorite in metamorphic fluids. Accordingly, their ages correspond to retrograde metamorphism and postdate peak metamorphism. The migmatitic paragneiss and leucogranite record at least three migmatization events at～1.85Ga,～830Ma and～660Ma, respectively. The～1.85Ga migmatization probably resulted from in-situ partial melting during a regional high-grade metamorphism, whereas the～830Ma leucogranite were likely derived from partial melting of deep or distant crustal rocks, implying large-scale melt migration and infiltration. These two migmatization events may correspond to two regional orogenic events, whereas the third,～660Ma migmatization probably resulted from local partial melting due to intrusion of hot alkaline granitic magmas.In contrast, the mica schist and paragneiss in the Xishankou area probably only experienced greenschist facies metamorphism and were metamorphosed at～1.93Ga, coeval with the emplacement of～1.93-1.94Ga intruding granitoids. Abundant detrital zircons were dated at2.0-3.5Ga, with the youngest age peak at～2.05Ga, consistent with the detrital zircon record in several samples in the Korla area. These data indicate that the Xingditag Group was deposited between2.05and1.94Ga. A comparison with known magmatic records and zircon morphology analysis indicate that these detrital zircons were mainly derived from felsic igneous rocks in the northern Tarim Craton, and therefore can be used to constrain crustal evolution. These detrital zircons mostly exhibit high δ18O and negative εHf(t) values, suggesting an igneous source(s) mainly derived from reworking of high δ18O mature sediments. Linear regression analyses indicate that the oldest (3.7-3.9Ga) and youngest (2.78Ga) crustal components in the magma source have a remarkably consistent176Lu/177Hf ratio of～0.01. This value is significantly lower than that for mafic rocks but consistent with that for felsic rocks. These observations imply that3.7-3.9Ga ancient continental crust might have existed in the northern Tarim Craton, much older than previously thought, and might have been differented reworked as early as-3.5Ga by intracrustal remelting and reworking into modern-like mature continental crust.In addition, SHRIMP and LA-ICP-MS zircon dating indicates that the widespread late Paleoproterozoic granitoids in the Xishankou area were all emplaced at1.93-1.94Ga and metamorphosed at1.91-1.92Ga immediately following magma crystallization. But the emplacement age and metamorphic age are only resolvable by SHRIMP data. Zircon Hf-O isotopic modeling indicates that both mantle-derived juvenile magma and ancient continental crust were involved during magma genesis. The relatively K-rich monzogranite, quartz diorite/quartz monzonite and garnet-bearing granite were probably derived from partial melting of metasedimentary rocks (i.e., the Xingditag Group) due to intrusion of, and magma mixing with, mantle-derived mafic magma, whereas the relative Na-rich trondhjemite and tonalite were generated by partial melting of newly underplated mafic lower crust in the rutile eclogite facies (>50km) and were contaminated by sedimentary rocks at shallow depth. The1.94-1.91Ga magmatic-metamorphic event most likely occurred in an Andean-type continental arc, which was subsequently involved in continental collision at-1.85Ga. A compilation of available geochronological data indicates that two late Paleoproterozoic orogenic belts occurred along the northern and southern margins of the Tarim Craton, a～1.9-1.8Ga North Tarim Orogen and a-2.0-1.9Ga South Tarim Orogen, which were similar to the Trans-North China Orogen and the Khondalite Belt of the North China Craton, respectively. Based on regional geological correlation, it is proposed that the Tarim-Dunhuang-Alxa-Yinshan Blocks might have been a coherent block during the Neoarchean to Paleoproterozoic, which collided with the Ordos Block and its western extension along the Khondalite Belt-South Tarim Orogen during2.0-1.9Ga and then collided along the Trans-North China Orogen and the North Tarim Orogen at～1.85, docking the Tarim and North China cratons into the Columbia supercontinent.The unmetamorphosed granitoids in the western Kuruktag area were emplaced during three episodes:mid-Neoproterozoic (ca.830-820Ma), late Neoproterozoic (ca.660-630Ma) and mid-Paleozoic (ca.420-400Ma). The mid-Neoproterozoic granitoids were mainly generated by low-temperature, fluid-present partial melting of Archean mafic lower crust, possibly in a continental arc setting. The late Neoproterozoic and mid-Paleozoic granitoids were probably both formed by magma mixing between juvenile and ancient crustal materials, but the former has higher melting temperature and low melting pressure probably in a back-arc rifting setting, whereas the latter has the opposite melting condition and was probably formed in a new continental arc setting. Combined with other petrological, geochronological and isotopic data from the northern Tarim Craton, a Neoproterozoic to Paleozoic long-lived (950-300Ma) accretionary orogenic model is proposed. This model involves an early phase of southward advancing-type accretion from the microcontinental blocks (at950-900Ma) in Tianshan to northern Tarim (at830-780 Ma), and a later phase of retreating-type accretion due to northward slab rollback and trench retreat that led to back-arc rifting (at780-600Ma) and opening of the South Tianshan Ocean. The bi-directional subduction of the South Tianshan Ocean (ca.460-400Ma) produced two sub-parallel magmatic arcs, namely the Yili-Central Tianshan Hanshan arcs to the north and the northern Tarim arc to the south. Our model extends the accretionary history in the southwestern CAOB from the Paleozoic back into the early Neoproterozoic and links it with the circum-Rodinia accretionary system, highlighting the symmetric accretionary systems relative to Tarim and Siberia.