Geochemical Characteristics And Tectonic Significance Of Caledonian--Indosinian Intrusive Rock In Nuoergong, Inner Mongolia

Northern Alxa, a part of the northern margin of north China platform, was selected as the study area of this paper, where intrusive rocks formed at late Paleozoic and Mesozoic developed widely. This paper aims to discuss the tectonic significance of the intrusive rocks by researching their geological characteristics, petrology, chronology and geochemical characteristics.There are many types of intrusive rocks in this study area. The type of Caledonian period rock is gneissic granodiorite, while the type of Variscan intrusive rocks mainly include gabbro, diorite, quartz diorite, tonalite, granodiorite,(like porphyritic) monzonitic granite, syenogranite and alkali-feldspar granite, forming as a sequence of the magma evolution from basic to acid. The type of Indo-china period epoch rock includes quartz-syenite and syenogranite.Data has been obtained through using the LA-ICP-MS zircon U- Pb. In middle Silurian, the average age of granodiorite is 424.6±2.1Ma.In early Permian, the average age of gabbro is 275.5±1.8Ma;Quartz diorite developed at 275.2±1.8Ma; tonalite developed at 288.71±0.94 Ma; Granodiorite developed at 278.1±1.2Ma; Monzonitic granite developed at 275.8±1.8Ma; syenogranite developed at 274.4±2.0Ma.The average age of Monzonitic granite of late Permian is 255.3±2.5 Ma; the age of Granite porphyry is 254.7±1.4 Ma.In late Triassic, syenogranite is concentrated in the time between 217.5-222.5 Ma, the tested average age is 220.3±1.7 Ma.Geochemical characteristics of the intrusive rocks show that the granodiorite of silurian in caledonian period is subaluminous and Calcium alkali series rock, showing weak negative anomaly of europium and belonging to the enrichment of light rare earth type; Trace elements are lean for Nb, Sr, Cr, Co, Ni and Ti, and enrich for Rb, Ba, Zr, Th, etc. Intrusive rocks of variscan are subaluminous and calcium alkali series rocks; rare earth distribution curve trends to the right; light rare earth are relatively concentrative; heavy rare earth are not developed; they are cognate magma evolution sequence. The trace elements are characterized by enriching in large ion lithophile elements of K and Ba, and lean for the high field strength elements of U, Nb, Ti, etc. Intrusive rocks of indosinian are aluminum to subaluminous and calcium alkaline to alkaline rock series, with strong europium negative anomaly and belonging to the type of enrichment of light rare earth. The trace element enriches in Rb, Th, La and Zr, and is lean for Ba, Nb, Sr, Ti, etc.Rocks in the study area have the characteristics of volcanic arc granite, belonging to the epicontinental magmatic arc granite formation in west section of northern north China plate. Combined with geological characteristics, petrological characteristics,chronology and the geochemical characteristics, it shows that north China platform and tarim plate belongs to the subduction stage during caledonian period and early variscan(> 425-275 Ma), late variscan belongs to the collision orogenic stage(275-254 Ma), and indosinian(> 250-220 ma) belongs to stretch stage after collision.