Neoproterozoic Tectonic Evolution Of The Bikou Terrane,northwestern Margin Of The Yangtze Block

The Bikou Terrane preserves abundant records of the Neoproterozoic magmatism,sedimentation,and deformation of the northwestern periphery of the Yangtze Block.It is considered as a natural laboratory for in-depth discussions on the Neoproterozoic tectonic evolution of the Yangtze Block.However,there is still a lack of clear understanding of the Neoproterozoic tectonic evolution process and dynamic mechanism of the Bikou Terrane.In response to this key scientific issue,this doctoral dissertation focuses on the key Precambrian rock units in the Bikou Terrane,with the Yudongzi Complex,meta-volcanic rocks of the Bikou Group,sedimentary sequences of the Hengdan Group,mafic-felsic plutons as the main research objects.Through comprehensive field investigation,petrology,petrography,geochronology and geochemistry studies,this doctoral dissertation attempts to 1)pinpoint the tectonic attribute of the Bikou Terrane,2)sort out the genetic characteristics and tectonic background of meta-volcanic rocks in the Bikou Group,3)explains the deposition time,source characteristics and tectonic attributes of the Hengdan Group,and 4)clarify the formation time,genetic mechanism and dynamic process of the key magmatism in the Bikou Terrane.By systematically summarizing regional geological data and comprehensively analyzing previously published results,this doctoral dissertation also preliminarily discusses the Neoproterozoic tectonic evolution process and dynamic mechanism of the Bikou Terrane.The main achievements and understandings of this doctoral dissertation are as follows:(1)The Bikou Terrane is an early Precambrian terrane along the northwestern margin of the Yangtze Block,with evolution history traced back to the Archean to Paleoproterozoic time.The TTG gneisses belong to typical Archean TTG-type rocks.They show obviously depleted zircon Hf isotopes(?_Hf(t)=+2.1-+8.1)and were originated from partial melting of the newly formed mafic crust,representing an initial crustal growth and reworking newly formed crust event at ca.2.82 Ga.The amphibole plagiogneisses were mantle-derived rock units with mainly depleted zircon Hf isotope(?_Hf(t)=-0.9-+3.9),representing an important crust growth event at ca.2.69 Ga.The geochemical composition of the granitic gneisses is also like Archean-type rocks.They exhibit a wide range of zircon Hf isotopes(?_Hf(t)=-3.4-+1.5)and were formed by partial melting of ancient crustal materials with a few mantle material additions,representing a reworking ancient crust event at ca.2.45 Ga.Moreover,the metamorphic age of the amphibolite represents a regional metamorphic overprint at ca.1.85 Ga.The rock composition and episodic tectonic-thermal evolution events of the Yudongzi Complex are relatively comparable to the crystalline basements within the Yangtze Block such as the Kongling Complex and the Zhongxiang Complex,indicating the potential affinity between the Yudongzi Complex and the Yangtze Block.(2)The Bikou Terrane was evolved into a tectonic setting related to sustained subduction accompany with slab rollback at least in the early Neoproterozoic at ca.880 Ma.The mafic intrusions in the Bikou Terrane,i.e.,the Huayangou gabbro-diorites,Linhouba gabbros,and Pingtoushan gabbros,were formed coevally at ca.880 Ma,which is the identified earliest Neoproterozoic magmatism in the Bikou Terrane.The Huayangou gabbro-diorites show typical arc magmatism's geochemical signals.These gabbro-diorites were resulting from partial melting of a lithospheric mantle wedge peridotite,which had been previously modified by subducted sediment-derived melt.Furthermore,the Linhouba gabbros and Pingtoushan gabbros display similar geochemical and isotopic characteristics,consistent with the composition patterns of a typical E-MORB,indicating that they were the product of partial melting of the E-MORB-type upwelling enriched asthenospheric mantle under reduced pressure.The Huayangou gabbro-diorites were formed in a subduction-related arc environment,whereas the Linhouba gabbros and Pingtoushan gabbros belonged to the magma response under the slab rollback mechanism of subducting oceanic crust.(3)The Bikou Terrane continued to be controlled by a subduction-related mechanism accompany with slab rollback in the early Neoproterozoic during ca.860-825 Ma.Among the felsic plutons in the Bikou Terrane,the Baiquesi quartz monzonites,Bahaihe quartz monzonites,and Shilingou monzogranites have similar crystallization ages at ca.860 Ma,whereas the Maliupu granodiorites show a little later formation age at ca.825 Ma.Compositionally,the Baiquesi quartz monzonites,Bahaihe quartz monzonites,and Shilingou monzogranites display similar isotopic characteristics and elemental evolution trends,while the monzonites were derived from differentiation and crystallization of the parent magma of quartz monzonites.The Baiquesi quartz monzonites and Bahaihe quartz monzonites show characteristics similar to adakitic rocks,with mantle-derived positive zircon Hf(?_Hf(t)=+4.8-+6.7)and whole-rock Nd(?_Nd(t)=+1.74-+2.09)isotopes.These quartz monzonites were most probably the product of the interaction between the subducted oceanic crust slab melts and the overlying mantle wedge peridotite,which were most probably formed by the continuous upwelling of the asthenospheric mantle material.In addition,the Maliupu granodiorites belong to typical I-type granitoids,with enriched zircon Hf(?_Hf(t)=-15.0--10.9)and whole-rock Nd(?_Nd(t)=-11.81--11.94)isotopes.These granodiorites were formed by partial melting of the ancient middle-lower continental crust,representing a significant reworking ancient crust event in the Bikou Terrane.(4)The continuous slab rollback beneath the Bikou Terrane caused the triggering of the back-arc extensional mechanism at ca.845-760 Ma.Based on geochemical characteristics,the meta-basic-intermediate volcanic rocks of the Bikou Group in the Bikou Terrane can be categorized as three groups:Group-I,Group-II,and Group-III.Group-I meta-basic-intermediate volcanic rocks are similar in composition to the IAB,which were originated from partial melting of the mantle wedge r metasomatized by subduction-related components;Group-II meta-basic volcanic rocks are similar to these of a typical E-MORB,indicating that they were derived from partial melting of the E-MORB-type upwelling enriched mantle material;Group-III meta-basic-intermediate volcanic rocks show characteristics similar to the OIB,implying they were derived from the OIB-like asthenospheric mantle with a small amount additions of crust-derived components.The meta-acid volcanic rocks in the Bikou Group also include two groups:Group-I and Group-II.Group-I meta-acid volcanic rocks have a wide range of Mg O,Ni,and Cr content,derived from partial melting of old lower crust with some mantle component additions;Group-II meta-acid volcanic rocks show low Mg O,Ni,and Cr content,formed by partial melting of ancient crustal materials.Both the meta-basic-intermediate volcanic rocks and the meta-acid volcanic rocks belonged to the magma response under the back-arc extensional system of the Bikou Terrane.(5)The tectono-thermal activity of the Bikou Terrane tended to be quiet during the middle Neoproterozoic after ca.720 Ma,whereas the tectonic regime gradually transitioned to the extensional rift setting.The clastic sedimentary sequences of the Hengdan Group in the Bikou Terrane are a set of proximal deposits rich in pyroclastic materials.Concordant dating results of the lower Baiyang Formation and upper Yangtianba Formation have similar maximum depositional ages at ca.720 Ma,indicating that the Hengdan Group belongs to a middle Neoproterozoic rapid accumulated sedimentary sequence.The bottom and upper parts of the Hengdan Group have similar provenance attributes.Detrital zircon age patterns of both the Baiyang Formation and the Yangtianba Formation are characterized by dominant Neoproterozoic(ca.915-720 Ma)and a small amount of Paleoproterozoic-Mesoproterozoic(ca.2450-1750 Ma)ages.The early Neoproterozoic rock units of the Bikou Terrane and the adjacent Hannan-Micangshan area may be the main source of the detritus.The bottom and upper parts of the Hengdan Group also have similar geochemical composition.Geochemical analysis reveals the compositions of the clastic rocks of the Hengdan Group have similar composition characteristics to the turbidites of the typical fore-arc basins.The Hengdan Group should be a product developed in a middle Neoproterozoic sedimentary basin in the Bikou Terrane with deposition time later than ca.720 Ma.(6)Based on the research achievements of this doctoral dissertation,combined with previously published research,this doctoral dissertation preliminarily proposes that the crystalline basement of the Bikou Terrane was formed during the Archean-Paleoproterozoic time.The Bikou Terrane belongs to the early Precambrian tectonic unit of the Yangtze Block.During the Neoproterozoic time,the tectonic activities of the Bikou Terrane tended to be activated,and the specific evolution process may mainly include the following four stages:magmatic arc development and slab rollback stage during ca.880-860 Ma in the early Neoproterozoic;continuous slab subduction,triggered back-arc extension,and limited back-arc rift basin development stage during ca.845-760 Ma in the early Neoproterozoic;tectonic regime transformation and magmatic activity quiet stage later than ca.720 Ma in the middle Neoproterozoic;magmatism calming down,rift-sag basin evolution and sedimentary cover development stage during the middle to late Neoproterozoic.