Research On Petrogenesis And Tectonic Affinity Of Different Rocks From The Jinjiahe Area In The Mianlue Tectonic Belt Of South Qinling

The Mianlue Tectonic Belt(MTB),as an important plate suture zone of the Qinling orogenic belt,is a crucial area to obtain the for Mation and evolution of Central China Orogenic Belt and the Chinese Mainland.At present,there are some academic disputes about the MTB,such as the closure time and tectonic affinity.These disputes Mainly result from a different understanding of rock assemblages,for Mation ages and tectonic attributes in the Mianlue Tectonic Belt.The Jinjiahe area is the representative exposed area with ideal Material constitution of Mianlue Tectonic Belt.In this research,the field geology,petrology,geochemistry,and isotope chronology of different types of Tectonic Blocks in the Jinjiahe area are syste Matically studied,and the tectonic setting and defor Mation character will be discussed in the following chapters.The Main results and progress achieved are as following:1.There is a wide range of"tectonic blocks"and"Matrix rocks"around the Sanchazi in the Jinjiahe area of the MTB.The rocks in these tectonic blocks include serpentine,meta-gabbro-diorite,meta-andesite,plagiogranite and weakly defor Mable rocks,whereas the Matrix rocks are strong defor Mable rocks such as Jinjiahe phyllite.The serpentinite,meta-gabbro-diorite,meta-andesite and plagiogranite are a set of broken SSZ ophiolite.The protolith rock of the serpentinite is harzburgite,which displays characteristics of SSZ-type Mantle peridotite.The meta-gabbro-diorite,meta-andesite and plagiogranite are characterized by island arc affinity.The zircons U-Pb ages(LA-ICP-MS)of the meta-andesite and the plagiogranite are 936 Ma and 880 Ma,respectively.This indicates that the suite of ophiolite is related to the subduction of the Mianlue basin during the early and middle Neoproterozoic.The Gounao granite in the Wenjiagou area is a high-K calc-alkaline I-type granite,which was formed in the post-collisional extensional stage during the Indosinian period and later emplaced in Mianlue Tectonic Belt.2.The phyllite in the Jinjiahe area is a part of the Guanjiagou For Mation(Nh2g)of the Nanhua system.We conducted LA-ICP-MS U-Pb dating on detrital zircons from the gravelly sericite-dolomite-quarts-phyllite and sericite-dolomite-quarts-phyllite of Jinjiahe phyllite.The obtained zircon U-Pb ages can be subdivided into three groups:Neoproterozoic age group(677?966 Ma),with peak ages of 781 Ma and 865 Ma;Mesoproterozoic age group(1122?1432 Ma);Palaeoproterozoic-Neoarchean age group(1944?2824 Ma).The minimum average ages of the detrital zircons in the two metamorphic clasticrocks are 725 Ma and 773 Ma,separately.It is concluded that the Main sources of the Jinjiahe phyllite are Neoproterozoic Mag Matic rocks in the MTB and the northwestern Margin of Yangtze Block,and the Maximum depositional age is not earlier than the Nanhua period.The siliceous rocks interbedded in the Jinjiahe-phyllite were biogenic and formed in the hypoxia-reducing environment in the middle and lower part of the continental Margin.The siliceous rocks can be compared with the black rocks of the Cambrian Linjiang For Mation.Thus the sedimentary period of siliceous rocks is Cambrian.3.Based on the comprehensive analysis of the Macroscopic structural defor Mation characteristics and microscopic structural characteristics of the tectonic blocks and Matrix rocks in Jinjiahe area of Mianlue Tectonic Belt,the defor Mation sequence of different rocks can be divided into four phases.The first phase formed the early structural plane(S₁)and syn-tectonic anatectic vein.The second phase is the top to south thrust shearing in the stage of plate thrust and extrusion.The third phase is the left ductile strike-slip shearing after the the continental collision.The fourth phase is the brittle defor Mation stage under shallow-surface tectonic layers.