The Magmatism Of The Paleozoic Basic-ultrabasic Rocks In Qimantag Area,Qinghai Province

Qimantag is one of the most advancing area for the mineral exploration in our country,and it`s the key region for a major breakthrough in ore prospecting.At the same time,there are abundant mineral resources contained in basic ultrabasic rocks and therefor to study on basic ultrabasic magmatism,for ore prospecting has very important significance.This paper summarize the previous data and through detailed field observation and indoor works,to study the ultrabasic mafic ultramafic rocks in Qimantag area of Qinghai in Paleozoic by using mineralogy,petrology and geochemistry,to discuss the formation age,background and material source of the basic-ultrabasic magma.The basic-ultrabasic rocks were found in the south of the main ridge of Qimantag mountain,which is located in East Kunlun orogenic belt accompanied by intensive magma activities.And the rocks are mainly composed of Serpentinizaed pyroxene troctolite and dark altered diorite.According to the zircon U-Pb ages data,The zircon U-Pb ages of 380.8±5.3Ma and 391.9±4.0Ma were obtained for the serpentinizaed pyroxene troctolite and dark altered diorite,suggesting their formation age was middle-late Devonian;By analyzing the main elements of the samples,the results show that the samples belong to subalkaline rocks of tholeiitic series;REE distribution revealed a light REE(LREE)enrichment pattern,the LREE/HREE value is between 3.24~5.30,the severity of REE fractionation medium,REE distribution curve showed right type LREE enrichment;With absence of Eu anomalies(?Eu=0.87~1.27),average value is 1.02,Ce basically no absence,it shows that the source of rock material is closely related to the mantle;In the spider diagram of trace element ratio,it enrichment of Rb,Th,U,Pb,Sr but depletion of Ba,Nb,Zr,Cr and Ni,other elements are not obvious.Analysis result of Sr-Nd-Pb isotope and zircon Hf isotope showed magmaderived from the mantle,which were contaminated by crustal materials obviously.The Rb/Sr value of the Serpentinizaed pyroxene troctolite are 0.10 and 0.09,the value of 143Nd/144 Nd are 0.512551 and 0.512681,?Nd(t)are 0.67 and 2.87,the Rb/Sr value of the dark altered diorite is 0.12,the value of 87Sr/86 Sr is 0.708138,?Sr(t)is 51.633096,?Nd(t)is-1.34,the ?Nd(t)value of the Serpentinizaed pyroxene troctolite is greater than 0,and the value of ?Nd(t)close to the earth and gross(?Nd(t)= 0),indicates that rocks are derived from weakly depleted mantle materials.The ?Sr(t)value of dark altered diorite is greater than zero and ?Nd(t)value close but less than zero,it is deduced that diorite in the diagenetic process,with the participation of crustal material,there may be crust contamination in different degrees on the rise in the process of magma.The Rb/Sr range of the serpentinizaed pyroxene troctolite and dark altered diorite values is 0.09~0.12,it is deduced that the magma is mainly composed of crust and mantle mixed source;Based on the Pb isotope data analyses,most of the samples are located in the contact zone between the orogenic belt and the crust,the Qm107-7 show the upper crust characteristics,it means the basic ultrabasic rock maybe have different source region or mixed the different region`s things;The middle Devonian mafic dyke may represents the change of Proto-Tethys Ocean ocean to the Paleo Tethys ocean,it is probably means the open of the Paleo Tethys ocean in eastern Kunlun orogenic belt in the Middle Devonian period.The bimodal volcanic rocks of the yak formation in late Devonian formed a good combination of relations with the basic ultrabasic rock samples we ware found this time.It is important to determine the tectonic environment in the area by combine the two analysis,it is suggested that Qimantag area was formed in the Middle-Late Devonian,the transformation stage from collision and compression system to the post-tectonic tension environment.Through comprehensive analysis,The Qimantag area was formed in the Middle-Late Devonian,the transformation stage from collision and compression system to the post-tectonic tension environment,the basic ultrabasic rocks were formed in the middle Late Devonian in this area,in the geological background of intracontinental extension.