The Palezoic Tectono-Magmatic Evolution Of The Northern Great Xing’an Range

The thesis deals with the systematic studies on mineralogy, petrology,geochemistry, isotopic geology and geochronology of the ultramafic-mafic rocks fromthe Xinlin-Gaxian-Jifeng ophiolitic mélanges, the volcanic rocks from theBoketu-Moguqi areas, and the sedimentary rocks from the Yinmin area in the northernGreat Xing’an Range, respectively. The Petrogensis, nature and emplaced setting ofthese ophiolitic mélanges have been discussed, and considered to be possibly relatedto the convergence and breakup of the Rodinia supercontinent. In comparison with theCarboniferous magmatisms in the western and eastern margin of the Xing’an block. Apreliminary Palezoic tectonic-magmatic evolution model for the northern GreatXing’an Range has also been put forward.The Xinlin-Gaxian-Jifeng ophiolitic mélanges (ultramafic rocks, metabasalts andgabbros) crop out in the northern Great Xing’an Range, the eastern segment of theCentral Asian Orogenic Belt, which marks the closure of the Xinlin-Xiguitu Oceanassociated with the collision between the Erguna and Xing’an blocks. Zircon U-Pbdating from these ophiolitic gabbros yields U-Pb ages of539-510Ma (Xinlin)，~630Ma (Gaxian),647±5.3Ma (Jifeng), respectively, which may represent the formationages of these ophiolitic mélanges. The Jifeng gabbros display low SiO2, TiO2, K2Ocontents, high Na2O, LREE contents and indistinctive REE fractionation. It shows anE-MORB-like affinity, while the element concentrations of the Jifeng samples arelower than that of E-MORB. Nb displays negative anomaly in comparison to Th,which shows a transitional SSZ-type ophiolite signature. Moreover, the εHf(t) valuesof~647Ma zircons in the gabbros range from+8.4to+13.4, and the corresponding Hf single-stage ages (TDM1) are between687and902Ma, which is obviously olderthan the crystallization age of647Ma. These geochemical features can be explainedas melts from the partial melting of a depleted mantle source metasomatized by fluidsderived from a subducted slab. In addition, the geochemical signature of the Gaxianand Xinlin ophiolitic mélange are similar to that of the E-MORB-like Jifeng gabbros.Meanwhile, new zircon U-Pb data and Hf isotope from Middle Devonian andLower Carboniferous units from NE China provide evidence for the depositional ageof these units as well as for the timing of the collision process between the Erguna andXing’an blocks. The detrital zircons from the Devonian and Carboniferous sandstonesyielded similar age spectra in probability diagrams showing three distinct peaks of~450Ma,~500Ma and~540Ma, respectively. Along with Proterozoic and Archeanage groups, except for younger population of~360Ma in the Hongshuiquanformation. These age groups are widely found in the Erguna and Xing’an blocks.Combined with the Hf isotope (-21~16) information from our sandstones andgranitoids investigated from these two blocks, we suggest that the Erguna andXing’an blocks started to accrete at~540Ma, reached the peak of collision at~500Ma.There are magmatisms related to the subduction of the northern branch(Nenjiang-heihe Ocean) of the Paleo-Asian Ocean after the collision between theErguna and Xing’an blocks, e.g. the Duobaoshan ore field, the Duobaoshan ultramaficrocks (~480Ma), basalt-andesites (447Ma) and the Yikete diorites (~430Ma).Significantly, the Carboniferous period marks a key transition from accretion tocollision in the northern Great Xing’an Range, NE China. The new age data from theTayuan and Jifeng consist of zircon SIMS U-Pb ages of306-315Ma and zirconLA-ICP-MS U-Pb ages of331Ma from basic intrusive rocks and329.4Ma fromgranites in the Erguna-Xing’an block. Zircons are euhedral-subhedral in shape,displaying straight rhythmic stripes and oscillatory zoning in CL images with Th/Uratios of0.20to3.44, which indicate that the zircons are of igneous origin. Thegeochemical data suggest that these rocks were likely derived from partial melting ofmantle source with minor spinel metasomatized by slab-derived fluids. Taking into account of the field relationships, geochemistry and geochronologypresented in this thesis, combining with the previous reported regional geologicalevidence, a new model can be proposed for the Palezoic evolution in the northernGreat Xing’an Range:(1) In early Neoproterozoic (~850Ma), the breakup of theRodinia supercontinent, caused the bimodal volcanic rocks with ages of ca.851~761Ma and the depositional of the Ergunahe Group (795Ma~738Ma) developed in thenorthern Great Xing’an Range. The Xinlin-xiguitu Ocean initially formed between theErguna and Xing’an blocks.(2)~650Ma, the ocean crust of the Xinlin-xiguitu Oceanpossibly started to subduct in an intra-oceanic environment. As represented by the ca.647~540Ma ophiolitic mélanges with E-MORB-like along the Xinlin-Gaxian-Jifengareas.(3) In the period ca.540-500Ma, the subduction and subsequent collisionbetween the Erguna and Xing’an blocks, caused emplacement of theXinlin-Gaxian-Jifeng ophiolites and formation of the Toudaoqiao blueschsits.(4) Upto~500Ma, the exhaustion of the Xinlin oceanic crust led to collision between theErguna and Xing’an blocks, and hereafter formed the Tahe A-type granites with agesof~480Ma at a post-collisional setting.(5) In the period~480-430Ma, the Niqiuheformation and Hongshuiquan formation sediment were widely deposited in theYinmin area. The volcanic-magmatic arc along the Duobaoshan-Yiershi area wereconsequently succeeded by the subduction and extension with the orogenic granitoids(ca.480~350Ma) intruded near the southeastern margin of the Xing’an block.(6)Subsequent to the period of320Ma, the marine strata from the early stage of EarlyPalezoic disappeared and the continental facies strata from the late stage of the EarlyPalezoic were wildely explosed in the northern Great Xing’an Range. TheCarboniferous intrusive rocks (Tayuan-Jinfeng) formed through the partial melting ofa metasomatized subcontinental lithospheric in a subduction to post-collisionalextensional setting triggered by the upwelling of the asthenosphere generated by slabbreak-off of the subducted oceanic crust between the Erguna-Xing’an and Songnenblocks. The Erguna-Xing’an and Songnen blocks have been amalgamated in the endof Early Carboniferous.