Petrogenesis Of Metamorphosed Mesoproterozoic- Neoproterozoic Rocks In The North Wulan Terrane, Southern Qilian Orogen And Its Tectonic Implications

Identification and understanding the origin,tectonic evolution and affinity of the microcontinent or terrane involved in the orogen could not only provide important constraints on the overall architecture and evolution of an orogenic belt,but also aid in reconstruction of ancient continents regionally and related supercontinents worldwide.The North Wulan terrane is a newly identified metamorphosed terrane in the north of Wulan County,southwest of the Qilian Orogen,which is considered as part of the Qilian Block in an early Precambrian period.The terrane is composed mainly of three tectono-lithological units including the early Mesoproterozoic unit,the late Mesoproterozoic to early Neoproterozoic unit and the early Paleozoic unit.This thesis conducted an integrated study on the Mesoproterozoic to early Neoproterozoic metamorphic,magmatic and sedimentary rocks from the North Wulan terrane to reveal their component and formation age,source nature,tectonic setting and metamorphic process,which further provides new insights into the early tectonic evolution and continental affinity of the Qilian Block and its links to the evolution of the Columbia and Rodinia supercontinents.The early Mesoproterozoic unit crops out in the middle part of the North Wulan terrane and consists of dominant Al-rich pelitic gneisses and schists as well as felsic gneisses,quartzites,arkosites,amphibolites,granulites,calc-silicate rocks and minor marbles,intruded by¹.5 Ga trondhjemite,¹.1 Ga mega-porphyritic granodiorite and¹.0 Ga monzogranite.This unit experienced amphibolite-facies metamorphism,locally up to granulite-facies with pervasively developed anatectic leucosomes.The metasedimentary rocks can be divided into geochemically Al-rich and Si-rich rocks.The protoliths of Al-rich rocks are shales;while the Si-rich rocks are greywackes and arkoses.Al-rich rocks were mainly derived from immature source rocks dominated by first-cycle andesitic and granitic materials.Si-rich rocks show weakly to moderately compositional maturity with significant influx of felsic rocks and recycled materials in the source.Detrital zircon age spectra from the arkosite and mica quartz schist samples yielded consistent age spectra,which is characterized by two main age populations of2685–2276 Ma and 2098–1761 Ma peaking at².57 Ga,2.48 Ga,2.35 Ga,2.09 Ga,1.97 Ga and 1.86 Ga with mostly negative?_Hf?t?values?–14.0 to+3.6?.The deposition of this unit is constrained between¹.67 and 1.5 Ga by the youngest detrital zircon ages and the¹.5 Ga trondhjemitic intrusions.Combined with lithological associations and detrital zircon features,the deposition of this unit is consistent with a heterogeneous,proximal and fault-dominant continental rifting environment.Based on the U–Pb ages and Hf isotopes from adjacent cratons or blocks,the Tarim Craton is considered as the most likely source area.The early Mesoproterozoic continental rifting strata were intruded by a suite of fine-grained gray gneisses with LA-ICP-MS magmatic zircon dating of 1519±5 Ma and 1497±8 Ma.The gray gneisses consist mainly of biotite,plagioclase and quartz with minor K-feldspar.The sillimanite occasionally occurred attributed to contamination.The rocks have high SiO₂?70.6?75.6 wt.%?and Na₂O?3.96?4.84 wt.%?,but relatively low Al₂O₃?mostly<15 wt.%?and K₂O?0.90?2.52 wt.%?contents.They possess low contents of Ba,Ta,Nb,Sr,P and Ti but moderate Y on the primitive mantle-normalized spider diagram.On the chondrite-normalized REE diagram,the studied samples show slightly enriched LREE?La/Yb?_N=3.46?11.3)and flat HREE?Gd/Yb?_N=0.95?2.06)patterns with strongly negative Eu anomalies?Eu/Eu*=0.22?0.38?.Zircons from these rocks yielded?Hf?t?and T_DM2 values of-2.2–5.1 and1.93–2.39 Ga,respectively.Above all,these rocks have trondhjemite compositions and are comparable to the low-Al or low-pressure TTG rocks worldwide.The precursor low-Al trondhjemite magma was generated from partial melting of the early Paleoproterozoic amphibolites at a shallow crust level through possible heat input from upwelling mantle in a continental rifting setting.The early Mesoproterozoic unit experienced granulite-facies metamorphism in the middle to late Mesoproterozoic.The mineral assemblages and microtextures from three representative samples of garnet-rich semipelitic,mica pelitic and garnet-rich quartzo-feldspathic rocks indicate the development of three mineral generations as a whole.The first generation is well preserved in the garnet-rich semipelitic rock and characterized by inclusion minerals in the garnet porphyroblast and sillimanite of biotite₀+cordierite₀±sillimanite₀±K-feldspar₀±spinel₀±corundum±quartz?M0?.This mineral generation represents an early stage metamorphic event with P–T condition of T>750°C and P<6 kbar.The second generation is characterized by peak-stage minerals of garnet porphyroblasts and matrix assemblages of sillimanite₁+K-feldspar₁+plagioclase₁+biotite₁+quartz?M1?under an estimated condition of T=750–850°C and P=4–8 kbar.The third generation reflects a retrograde process characterized by reaction rims or symplectites around the garnets.The mineral assemblages consist of anorthite-rich plagioclase corona/cordierite₂+biotite₂±cummingtonite/grunerite±plagioclase₂±quartz,and the symplectite of anorthite₂+spinel₂+cordierite₂,as well as single biotite₂ and muscovite?M2?at T=560–610°C and P=2.8–3.6 kbar.The variations of mineral assemblages and compositions from the same mineral generation in different samples are controlled by the bulk rock compositions or chemical compositions of the reactant minerals.The reconstructed P–T path displays a clockwise feature with low pressure and high temperature,indicating an arc-continent collisional environment.Monazite dating was conducted on three representative samples.Combined with zoning patterns,index mineral inclusions,textual positions and chemical compositions,the early stage metamorphism is constrained to occur at¹.3 Ga,followed by arc-continent collision-related peak and retrograde processes at¹.1 Ga.The late Mesoproterozoic to early Neoproterozoic unit is exposed on both the northeast and southwest side of the early Mesoproterozoic unit bounded by thrust faults and consists mainly of felsic gneisses,quartzites,arkosite,metavolcanic rocks,amphibolites and calc-silicate rocks.This unit is characterized by large volumes of thick-layered marbles and<⁰.9 Ga garnet-bearing granites,and suffered from amphibolite-facies metamorphism.One metavolcanic rock yielded a LA-ICP-MS U–Pb age of¹110 Ma with positive?_Hf?t?values of+6.5 to+9.1.On the other hand,three metasedimentary rocks yielded detrital zircon ages clustering in the Mesoproterozoic to early Neoproterozoic with peaks at ca.1.64 Ga,1.43 Ga,1.3–1.2 Ga,1.1 Ga and 0.94 Ga with predominantly positive?_Hf?t?values ranging from–7.1 to+9.7.Considering the⁰.9 Ga garnet-bearing granitic intrusions,age of metavolcanic rock and the minimum age of detrital zircons,this unit is probably composed of two sub-units with depositional ages of¹.1 Ga and⁰.93–0.9 Ga,respectively.The detritus of sediments in this unit were mainly derived from proximal felsic to intermediate igneous rocks with the addition of recycled materials.The¹.1–0.94 Ga detritus were probably sourced from local igneous rocks from the early Mesoproterozoic unit;while the older sediments were probably derived from adjoining North America?Laurentia?.The Central Qilian and Hualong terranes show strong affinity with the North Wulan terrane and constitute a single coherent Qilian Block in an early time.Combined with previous works in adjacent regions,a tectonic model is proposed for the Qilian Block in the Mesoproterozoic to early Neoproterozoic period.The Qilian Block was once part of the Tarim Craton.During the breakup of the supercontinent Columbia,an initial continental rifting basin was developed on the pro-Tarim-Qilian continent with the deposition of¹.67–1.5 Ga terrigenous clasts,which was further intruded by voluminous¹.5–1.4 Ga anorogenic magmatism.The continent was finally separated into the North Tarim and South Tarim-Qilian terranes.Multiple arc-continent or continent-continent collisions were involved in the South Tarim-Qilian continent during a prolonged period of¹.3?0.9 Ga corresponding to the assembly of Rodinia.The early-stage sediments experienced two episodes of upper amphibolite to granulite-facies metamorphism at¹.3?1.1 Ga,with the arc-continent collision at¹.1 Ga.During¹.1?0.9 Ga,the continent was continuously under an active continental margin with the development of voluminous arc-related granitic intrusions and volcanic rocks.A suite of¹.1?0.9 Ga sediments were deposited on the subducted oceanic lithosphere with source materials both from local arc-related igneous rocks and older detritus from adjacent Laurentia continent.The subduction lasted until⁰.9 Ga possibly punctuated by continental collision to form a reunified Tarim-Qilian continent.The Tarim-Qilian continent together with Qaidam-East Kunlun,North Qinling,Alxa and microcontinents within the Central Asia Orogenic Belt?CAOB?may constitute a once larger continental domain at the early Neoproterozoic.