Paleoproterozoic Magmatism And Metamorphism Of The Western Quanji Massif,NW China And Implications For Tectonic Evolution

The Quanji Massif,located in the northwest China,is a cratonic continental remnant.It is composed of a basement consists of Paleoproterozoic medium-to high-grade metamorphic rocks and unmetamorphosed cover of Mesoproterozoic and Neoproterozoic strata,thus is similar to the North China Craton.Previous studies revealed that the Massif developed post-collisional granitoid magmatism during the early Paleoproterozoic and two episodes of tectonothermal event in the late Paleoproterozoic.However,it remains unclear about the time-span for the post-collisional granitoid magmatism and the whole event chain of the first episode.This thesis carries a comprehensively study in geology,petrography,mineralogy,geochronology and geochemistry on the three lithological units,i.e.,the meta-granitoid stock unit,meta-mafic dyke unit and syenogranite dyke unit of the basement in the Quanjishan area,western Quanji Massif,to reveal the occurrences,rock types,intrusive ages,petrogenesis,mantle source nature and tectonic settings,and preliminary study of metamorphic characteristics of the meta-mafic dyke unit.The study not only reconstructs the whole tectonic-thermal event chain of the first tectonothermal event in the late Paleoproterozoic,but also provides new constraints for plate tectonic operation regime and paleocontinent evolution in China and worldwide during the Paleoproterozoic.The Quanjishan meta-granitoid stock unit is a 2.37–2.34 Ga metamorphic intermediate-acidic intrusive rock suite,which constitutes predominant part of the Quanjishan basement.Their protoliths include monzodiorite,monzonite,quartz diorite,granodiorite and tonalite.These rocks show metaluminous to weakly peraluminous and correspond to high potassium calc-alkaline I-type granitoids with minor showing A₂-type granitoid affinities,and some of them have adakitic geochemical features.Their low MgO?1.37–4.09 wt.%?,Cr?7.66–79.2 ppm?,Ni?2.34–19.3 ppm?,various Sr/Y ratios?11.5–4.09?and Nd-Hf isotopes(?Nd?t?=–2.1⁺2.3,T_DM2=2.57–2.92Ga;?Hf?t?=-1.8⁺2.3,T_DM2=2.76–3.02 Ga)suggest that these granitoids were sourced from Mesoarchean to Neoarchean metamafic rocks in various depths of a thickened lower crust.These meta-granitoids show compositional complexity and diversity due to their multiple magma sources and partial melting depth levels for the magmas,which are consistent with the productions of post-collisional magmatism that were derived from various magma sources in post-collisional stage.The².37–2.34Ga granitoid suite intruded immediately following the².39–2.37 Ga granitoids in the central-eastern Quanji Massif and they are altogether of post-collisional magmatism,suggesting a ca.50 Myr or more protracted post-collision magmatism process.The meta-mafic dyke unit occurred as dykes?some of them dismembered as tectonic lenses by later deformation?in the Quanjishan meta-granitoid stock unit and consists of amphibolites,meta-gabbros and meta-diabases?some transformed into granulites?.The precursor magmas of their protoliths were formed at 1.95–1.94 Ga,and show subalkaline basaltic components with compositional complexity of tholeiite,transitional and calc-alkali series.These mafic rocks display lower MgO?3.02–6.86wt.%?,Ni?5.34–90.4 ppm?and Cr?6.71–169 ppm?contents than the mantle-derived primary melts,suggesting their precursor magmas had experienced crystalline differentiation with various degrees.Only those samples showing negligible crustal assimilation are chosen for evaluating magma sources,and their trace and rare earth elements present various degrees of enriched LILEs and LREEs and show remarkable Nb-Ta negative anomalies,in conjunction with their complex Hf isotopes(?Hf=10.3-4.6,T_DM=1.94–2.16 Ga;and?Hf=-3.3^-0.8,T_DM=2.37–2.46 Ga),which suggest their magma sources could be a lithospheric mantle together with an enriched asthenosphere mantle modified by subduction fluid.The mafic dykes underwent 1.93–1.92 Ga regional metamorphism shortly after intrusion.The peak upper amphibolite-facies stage mineral assemblages are brown-green hornblende+clinopyroxene+plagioclase,and some metadiabases develop orthopyroxene?hypersthene?+clinopyroxene mineral assemblage and then retrograde amphibolite-facies condition after the peak metamorphism.The peak temperature-pressure conditions of the upper amphibolite-facies and amphibolitic granulite facies are 780–800?and 8 kbar,suggesting a medium-P/T type regional metamorphism and collision-related,consitent with previous study results in the middle and eastern Quanji Massif.Given that these meta-mafic dykes are formed in the regional convergent tectonic background and show arc magma geochemical characteristics,composite mantle-derived properties,back-arc extension properties,as well as a collision-related medium-P/T type regional metamorphism shortly after intrusion,thus a back-arc basin on the active continental margin can be a reasonably tectonic setting for their precursor magma generation.Furthermore,the medium-P/T type regional metamorphism that affected the dykes could occur at arc-continent collision process.The syenogranite dyke unit emplaced into the Quanjishan meta-granitoid stock unit and crosscut the gneissosity of the stock and meta-mafic dyke units.The syenogranites can be subdivided into gneissic syenogranites and massive syenogranites according to their structural type,suggesting the former intruded earlier than the later.The gneissic syenogranite and massive syenogranite intruded at 1.90–1.87 Ga and1.90–1.86 Ga as evidenced by zircon U-Pb ages,respectively.But the two type syenogranites both are mainly made up of potassium feldspar and quartz with minor biotite.Geochemically they show metaluminous to weakly peraluminous high-differentiation granite affinity with high SiO₂?67.99–76.99 wt.%?,high K₂O?5.28–7.83 wt.%?and alkali-rich?K₂O+Na₂O=8.67–11.34 wt.%?.These syenogranites show the genetic type of I-and A-type granitoid affinities,and most rocks display C-type adakitic geochemical characteristics.Their magmas are derived from the reworking of the Mesoarchean to Neoarchean crust with highly evolved Hf isotopes(?Hf?t?=–7.9^–1.3 and T_DM2=2.71–3.04 Ga),potentially sharing same/similar magma sources with their country rock,the Quanjishan meta-granitoid stock unit,according to similar Hf isotope compositions between them.The syenogranite dyke unit closely intruded after the 1.93–1.92 Ga collision-related medium-P/T type metamorphism,suggesting in the post-collisional stage.The new data of this thesis fully reconstruct the key magmatic and metamorphic event of the event-chain of the first tectonothermal episode in the late Paleoproterozoic tectonic evolution in the Quanji Massif,and provide important implications for the global plate operating style in the early Paleoproterozoic age-trough between 2.45 and2.20 Ga and the assembly process of the Columbia supercontinent:?1?There occurred a plate subduction of⁷0Myr between 2.44 and 2.37 Ga and a protracted post-collisional magmatism corresponding to a plate subduction showdown or shutdown period of⁵0Myr between 2.39 and 2.34 Ga,suggesting that plate subduction operation and shutdown occurred successively within the 2.45–2.20 Ga presented by zircon age-trough in the Quanji Massif region.This new understanding is consistent with some coeval magmatic and tectonic events in the North China Craton and elsewhere globally,suggesting that plate tectonics operated in episodic subduction and stagnation rather than continuous subduction or complete stagnation in the entire early Paleoproterozoic period.?2?Time-span of the two episodes of subduction-accretion-collision-post collision extension processes in the Quanji Massif in the late Paleoproterozoic can be further constrain at¹.96–1.86 Ga and¹.86–1.77 Ga,respectively.These two episodes ocuurred coevally with those episodes for the final amalgamation and cratonization of the North China Craton,and also responded to the multistage assembly process of the global Columbia supercontinent.It is suggested that the Quanji Massif might be incorporated into the North China Craton and then being a part of the Columbia supercontinent through above assembly processes in the late Paleoproterozoic according to comprehensive comparison of magmatic,metamorphic and sedimentary events documented within basements of both the Quanji Massif and southwestern margin of the North China Craton.