Characteristics And Discussion Of Tectonic Environment And Genesis Of Igneous Rocks In The Ritu Area, Tibet

In the article, the author simply introduces such geological backgrounds as regional strata, tectonic units and main histories of evolution, and describes in details such magmatic features as geology, petrology, geochemistry of rock and accessory minerals , and discusses the forming ages and tectonic environments of the magma , and inquires into the emplacement mechanisms of granites and analyses by comparison the characteristics of intermediate-acid intrusive rocks in different tectonic units in the Ritu area, Tibet.The ophiolite in the Ritu area belongs to the western sector of the Bangon Lake-Ru River Ophiolite Belt. It is made up of such eight ones as Ritu Rock Body, Baerqiong Rock Body, and so on, distributing in NWW trend on the whole. It is also composed of such rocks as ultramafic rocks, accumulative rocks , gabbro wall groups .volcanic rocks, tuffs and siliceous rocks. Its primitive sequences were so broken down by strong tectonic activities that faults, as contact relations, exist extensively between rock units and between rock unit and its country rock. In the ophiolite, the ultramafic rocks consist of mostly harzburgite, less dunite, lherzolite, olivine pyroxenite, and the accumulative rocks include harzburgite and layered hornblende gabbro, and the volcanic rocks refer to basalt, picritic basalt and spilite.The ultrabasic rocks are attributed to magnesian ultrabasic rocks, the basic rocks mostly to rich-ferrobasic rocks, less to ferrobasic rocks, and the volcanic rocks to rich-ferrobasic rocks or to ferro-ultrabasic rocks. They belong to the Calc-alkaline series or the Tholeiitic series but the volcanic rocks, being more complex, to the Alkali basalt series or to the Calc-alkaline series or the Tholeiitic seriesIn spite of totally being low, their REEs have an increasing trend and their magmatic differentiations are gradually improved from ultramafic rock to basic rock to volcanic rock in the ophiolite. That and their isotopic features show that their magmas derived from the depleted upper mantle The ophiolite in the area is also thought formed mainly in an environment of mid-oceanic ridge of Tethys as they are the relics of the subduction of Bnagong Lake-Nu River Meso-Tethys and in geochemistry they are basically similar to the ones in the south Yarlungzangbojiang Ophiolite Belt and to the ones in the east Changling-Menglian Ophiolite Belt and their volcanic rocks are chiefly attributed to MORB or OIB.In the ophiolite, the siliceous rocks abound with radiolarites of Jurassic Period to Early Cretaceous Epoch. The country rocks of the ophiolite contain a sporopollen assemblage of Early-Meso-Jurassic Epoch and more animal fossils of Late Palaeozoic Era and of Jurassic Period to Cretaceous Period.The spilitc basalt have an isotopic age of 102.5±2.9Ma and the "inner ocean type "of shear zone has an age of 159.5±1.6Ma in the ophiolite.The ophiolite are overlaid by the Shamuluo Formation of Late Jurassic Epoch to Early Cretaceous Period, partly by the Jingzhushan Formation of Late Cretaceous Period in unconformity.In the north is the Late...