The Continental Crust Formation And Evolution Of The Beishan Orogenic Belt

As the last accretionary segment of the southern Central Asian Orogenic Belt(CAOB),the Beishan Orogenic Belt locates in the tectonic junction between the North China and Tarim cratons and bridges the South Tianshan and Solonker sutures.Thus,it provides well constrains on the amalgamation,evolution and final stabilization of the southern CAOB.The Beishan Orogenic Belt suffered from multiple magmaticmetamorphic episodes since early Neoproterozoic,resulting in a well-preserved magmatic,metamorphic and sedimentary sequence.In this contribution,detailed field investigation,systematic petrological,zircon U-Pb dating and Hf isotopes as well as whole-rock geochemical analysis were conducted on igneous rocks aging from Mesoproterozoic to late Paleozoic within this orogenic belt.Then,we have determined 1)the distribution of Precambrian plutons,their origins and possible linkage with Rodinia and Nuna supercontinents;2)the temporal and spatial distribution of Paleozoic magmatic rocks.Also,genesis and geodynamic settings of magmatic rocks from different locations and backgrounds as well as their implications for crustal and tectonic evolution of this orogenic belt were detailed discussed.High-precision LA-ICP-MS zircon dating results reveal that the tectono-thermal events in the Beishan Orogenic Belt dominated on Proterozoic and Phanerozoic.Granitic gneiss from Gubaoquan and Yadan regions yield 206Pb/238 U magmatic crystallization ages of 933 ± 2 Ma and 890-900,respectively.Their magmatic zircons always contain inherited cores with Mesoproterozoic ages.Meanwhile,five granitoids from the Jiujing region yielded igneous zircon 206Pb/238 U ages of 1450-1401 Ma,and zircon xenocrysts from late Paleozoic plutons in the Jiujing,Qiaowan,and Shibandun regions have U-Pb ages that also cluster at ~1.4 Ga.Those data suggest that crust of this age is widespread beneath the southern Beishan Orogenic Belt(SBOB).Besides,the ~1.4 Ga zircons display depleted Hf isotopic signatures(?Hf(t)= +2.0-+11.1)and show relatively young two-stage Hf model ages(1.9-1.4 Ga),reflecting remarkable crustal growth event.In contrast,zircon ?Hf(t)(magmatic rim)values range from-16.1 to +10.2,implying intensive reworking of pre-existing curst during Neoproterozoic.Thus,we consider the basement beneath southern Beishan Orogenic Belt generated during Mesoproterozoic and consolidated during Neoproterozoic.On the other hand,distinct Precambrian tectonothermal history between the SBOB,nearby Dunhuang block and the Tarim craton argues against their tectonic affinity,instead it perhaps has a common affinity with the Central Tianshan Arc.The Neoproterozoic gneisses belong to the high-K,calc-alkaline series and are characterized by an enrichment of light rare earth elements(LREE)and large ion lithophile elements(LILE),a depletion of Nb,Ta,Ti,Sr,and Ba and a positive Pb anomaly.The petrography and geochemical signatures reveal a possible I-type granite affinity and are in accordance with typical Andean Arc granites.Combined with coeval orogenic granulite facies metamorphism,we consider the ca.900 Ma plutons generated in an Andean-type active continental arc setting during the assembly of Rodinia.Furthermore,we find Mesoproterozoic granitoids from microcontinents(including the SBOB)within the southern CAOB display distinct geochemical signatures(e.g.,Ga/Al*10,000 ranging from 1.46 to 3.88 and Zr+Nb+Ce+Y varying largely from 101 to 1066 ppm)and show a temporal progression from orogenic to anorogenic over a relatively short period.This suggests a tectonic setting that likely evolved from compressional to extensional environments during 1450-1360 Ma,perhaps resulting from slab roll back process.We suggest that the 1450-1360 Ma magmatism along the southern CAOB corresponds with a transition from a compressional to extensional environment on the margin of the Nuna supercontinent.Early Paleozoic magmatic rocks is pervasive in the Beishan Orogenic Belt,they are usually granitic in composition and suffered from intense metamorphism and deformation.The early Paleozoic granitoids from the Hongliuxia granite complex(446-430 Ma)of the Mazongshan unit and the Shibanjing complex(464-428 Ma)of the Hanshan unit show classic I-type granite affinity as revealed by the relative enrichment of LILEs and LREEs,pronounced depletions of Nb,Ta and Ti and the abundant presence of hornblende.Furthermore,they are characterized by strongly variable zircon ?Hf(t)values between-17.2 and +12.5 and evolved plagioclase Sr isotopic compositions of 0.7145-0.7253,indicating the involvement of both juvenile(minor)and ancient continental crust in the magma source.Thus,we propose that the early Paleozoic granitoids in the northern Beishan Orogenic Belt(NBOB)were generated in a subduction-related compressional setting.In contrast,the late Paleozoic 330-281 Ma granitoids from the Shuangjingzi complex of the Hanshan unit show less differentiation of REE and negligible deformation than the early Paleozoic ones and exhibit positive zircon ?Hf(t)values between +5.4 and +12.8 as well as relatively depleted plagioclase Sr isotopic compositions of 0.7037-0.7072,indicating that they were mainly formed by remelting of juvenile crust in an extensional setting.Therefore,between the early and late Paleozoic,the magma sources of the NBOB granitoids converted from the reworking of both juvenile and ancient crusts in a subduction-induced compressional setting to the remelting of juvenile crust in an extensional setting,respectively.The corresponding crustal growth is dominated by early Paleozoic lateral accretion of arc complexes and late Paleozoic vertical addition of juvenile material from the mantle.The Mazongshan and Huaniushan units in the central and south Beishan Orogenic Belt developed early Devonian A-type granites(404-403 Ma and 411-399 Ma,respectively),they reveal depleted zircon-Hf isotopic signatures(?Hf(t)= +4.5 to +11.1 and-2.9 to +5.7 for Huaniushan and Mazongshan samples,respectively),corresponding with moderate whole-rock Nd isotopes(?Nd(t)=-0.2 to +0.5 and-2.7 to-2.2).The Mazongshan samples have slightly older zircon-Hf and whole-rock Nd model ages than the coeval Huaniushan samples,indicating a higher proportion of ancient crustal component in the magma source,which argues against its traditionally inferred intraoceanic setting.Given that 1)the Mazongshan and Huaniushan granites display a marked rise in their Ga/Al ratio during the early Devonian;2)the late Paleozoic magmatic records of the Beishan Orogenic Belt,which contains 374-365 Ma adakites,290-270 Ma postcollision granites,rift-related basalts or mantle plume-related mafic plutons/dykes,as well as bimodal volcanic associations,all of which are distinct from the early Paleozoic arc-related magmatic records;and 3)the Cambrian to Silurian sedimentary strata of the Huaniushan and Mazongshan units are mainly composed of fore-arc volcanoclastic rocks,limestone with subordinate sandstone and mudstone,whereas the unconformably overlying middle Devonian strata indicate a change to terrestrial deposits that perhaps accumulated in an extension-related back-arc setting.Thus,igneous activity occurred in a compressional setting prior to early Devonian and in an extensional situation for the remainder of the Paleozoic,which is interpreted to mark the switch from an advancing to a retreating accretionary orogen.Zircon U-Pb dating results reveal that two phases of magmatism with the older from the late Ordovician to early Silurian(455-435 Ma)and the younger in the early Permian(296-274 Ma)occurred in the abutting region between the southern Beishan Orogenic Belt and Dunhuang block.The former is dominated by intermediate to acidic rocks(Si O2 = 54.1-74.9 wt.%),whereas the latter is characterized by a large variation of Si O2 contents(46.4-72.6 wt.%).They have a similar petrogenesis with the NBOB Paleozoic igneous rocks as mentioned above,namely the magma sources of the early and late Paleozoic plutons changed from relatively deep and acidic-intermediate compositions(ancient crust)to shallow but mafic compositions(juvenile crust).This change may be caused by the conversion from a compressional to an extensional setting.This abutting region has similar Paleozoic tectono-thermal evolution with the southern Beishan Orogenic Belt rather than the Dunhuang block.Moreover,the magmatism,crustal evolution and sedimentary records in the north and south of Sule river are distinct,thus we propose the boundary between the southern Beishan Orogenic Belt and the Dunhuang block lies along the Sule river.On the other hand,given the southern Beishan Orogenic Belt and Dunhuang block are characterized by early Paleozoic magmatic plutons and metamorphic rocks,respectively,we consider the former was structurally juxtaposed against the latter during early Paleozoic.This process dominated the tectonic evolution of the early Paleozoic SBOB,whereas the late Paleozoic SBOB might be attributed to the back-arc basin system.Given above,magmatic activities are widespread over Paleozoic in the Beishan Orogenic Belt and climaxed at Ordovician-Silurian and Carboniferous-Permian.During the early Paleozoic,southward subduction of Paleo-Asian ocean and the resultant interaction between the southern Beishan Orogenic Belt and Dunhuang block dominated the tectonic evolution in the north and south of this orogenic belt,respectively,resulting in abundant granitic plutons in the south Hanshan unit,Mazongshan unit,north Huaniushan unit and south Shibanshan unit.These products reveal large variation of zircon-Hf composition,which not only imply the intense reworking of ancient crust but also require variable addition of mantle-derived materials in their magma origin.Thus,the crust growth mechanism is dominated by horizontal accretion of complex arc during this stage.The early Devonian igneous activity is dominated by A-type granitic magmatism in the Huaniushan and Maozngshan units,its depleted zircon-Hf and wholerock Nd signatures indicate obvious crust growth in an extensional setting.Late Paleozoic magmatic activities dominated in Liuyuan volcanic belt(south)and Hongshishan melange(north).Besides,late Paleozoic dykes,intrusions and mafic enclaves are commonly hosted by pre-existing rocks.These rocks are predominated by depleted zircon-Hf and/or whole-rock Nd isotopes,imply noticeable addition of mantle-derived materials,perhaps by vertically under-plating of basaltic magma.In this stage,the north and south of Beishan Orogenic Belt probably operated by oceanic crust subduction and back-arc basin opening,respectively.