Geochemical Characteristics, Diagenetic Ages And Tectonic Significances Of The Intermediate-felsic Intrusive Rocks In Qinghai Lajishan Mountains

The Qilian microcontinent is a one of the ancient upwarping regions in Qilianshan orogenic belt. The south of it is Zongwulong mountain- Qinghai South Mountain Orogenic Belt, which is adjacent to Olongbluk microcontinent. And in the north it is separated from the Alashan microcontinent by the Northern Qilian orogenic belt. It is a Neoproterozoic ancient continental block with Paleozoic sedimentary stratum covers on it, and it has typical dualistic structure characteristic. Its basement rocks are made up of Early Proterozoic high grade metamorphic rocks and continental shelf sedimentary rocks from Neoproterozoic to Mesoproterozoic Era with cover layer properties. The Qilian microcontinent had been divided to Middle Qilian microcontinent and Southern Qilian microcontinent by Shulenanshan- Lajishan suture zone which runs northeast or almost east to west.The north and south sides of Lajishan suture zone are in fault contact with the cryptozoic crystalline basement. It has multi-periods activity attributes and with several violently tectonic movements in Paleozoic, Mesozoic, and Cenozoic. The main rocks inside is early Paleozoic(mainly Cambrian) volcanic rocks. And there is Lajishan ophiolitic mélange belt containing dunites, limburgites, pyroxenites gabbro-diabases and silicalites, and also a large number of intermediate-felsic intrusive rock bodies of plagiogranites, quartz diorites, granodiorites, diorites etc. This paper emphatically discussed the petrology, geochemistry, diagenetic age and tectonic significance of the three small intermediate-felsic intrusive rock bodies outcrops on the southern edge of Lajishan ophiolitic mélange belt.The three small intermediate-felsic rock bodies lie in the northwest peak, ghaut and the highest peak of Lajishan mountains respectively. They have a banding distribution along the northwest-southeast direction which is similar to Lajishan ophiolitic belt. The main rock type of the three rock bodies is diorite, then quartz diorite, granodiorite, with a large number of hornblende gabbro and pyroxene diorite inclusions, also some acid rock lenticles or dikes like monzonitic granite in them.The major element ingredient of the rocks in the three diorite rock bodies shows that they belong to metaluminous and calc-alkaline rock series. The trace element spider diagrams show they almost have heavy losses of Nb, Ta, Ti, and relative losses of Zr, Hf, which are island-arc basaltic magmatic crystalline rock’s characteristics. Sr,Nd isotopic ingredient reveals their diagenetic primary magmas were derived from thecrust-mantle transition zone, and mainly caused of mantle’s partial melting in lithosphere, with crustal material’s superimposition.Uniting the comprehensive research, we consider the genetic type of the three diorite rock bodies on the southern edge of Lajishan ophiolitic mélange belt is I type granites. And the rocks belong to calc-alkaline rock series. Combining the rare-earth elements and trace elements contents, we think they could have formed island-arc gabbro cumulus crystal as a result of island-arc basaltic magmas intruded upward,they are crystallization product of residual magmas with magma chamber peripheral crustal materials mixed into it. The LA-ICP-MS zircon U-Pb dating results reveal the rocks’ forming ages are about 460-465 Ma, which belongs to Middle Ordovician. The latest research about the genetic and the diagenetic ages of the three small intermediate-felsic intrusive rock bodies on the southern edge of Lajishan ophiolitic mélange belt reveal the tectonic regime of island arc subduction, oceanic crust reduction, and island-arc magmas intruded upward in Lajishan suture zone in Middle Ordovician.