Cause Of The Basic-ultra Basic Rocks In Southern Qiangtang,northern Tibet

Qiangtang massif is isolated from Gangdise massif in the south by BangongCo-Nujiang River suture zone, and connected to Lhasa and Songpan-Ganzi massifwith Jinshajiang suture zone. Karakoram massif in the west is the westward extensionof the Qiangtang massif..Qiangtang massif is divided into south and north two partsby Longmu Co-Shuanghu-Lancang suture zone. The study region is located in thecentral Qiangtang, included by stratigraphic subregion of southern Qiangtang. Incentral Qiangtang, exposed strata is discontinuous due to structural damage. However,ophiolite and high-pressure metamorphic belt remained intact in the study region.,constituting main component of the Longmu Co-Shuanghu-Lancang suture zone.Basic-ultrabasic rocks in southern Qiangtang mostly consist of basalt interbededin Late Palaeozoic strata.The maximal thickness of basalt can be more than onehundred metres. Picrite-porphyrite mostly sporadically together with basic dykes andbasalt outcrops in Gemuri, Pianshishan, Erlianhu and Mayigangri, south of LongmuCo-Shuanghu-Lancang suture zone.The rock is of small scale in the form of minorintrusions with a width range from ten meters to hundreds of metres, a length fromdozens of meters to several kilometers(northern Mayigangri). Picrite-porphyritemainly includes clinopyroxene-peridotites and lherzolite. The feature suggestsultrabasic rocks may be from the mantle plume. According to the geochemistryresearch done by us before, the ultrabasic rocks are meimechites and might have beenderived from garnet lherzolite of the mantle with little contamination of crustmaterials. The surrounding rocks of the basic-ultrabasic rocks mainly consisit ofconglomerate,sandstone, sandstone with limestone, oolites and glacial-marineconglomerate formed by Gondwana continental rift valley-type sedimentation.A map that shows the distribution of basic dykes is pictured. In this map,distribution area of basic dykes is more than80000km2with a range of700km from east to west, and150km from north to south. Considering that there arelarge areas of Mesozoic and Cenozoic strata in east of Shuanghu, the actual rangecan be extended eastwards. The densest distribution is in south of Guoganjianainmountain.To the east distribution density increases, more concentrated, and to thewest distribution area increase gradually while density decreases and spreadgradually, with the characteristics of the convergence from west to east. Theradiation angle is about30degrees, Convergence center is reflected to be locatedin Mayigangri, which may be the center of mantle plume.Zircon U–Pb isotope analyses indicate that the dykes were emplaced in the EarlyPermian (302±4Ma,284±3Ma,279±2Ma,285±3Ma,283±1Ma). The ages ofthe three diabase samples are300.0±1.8Ma,291±2Ma,292±3Ma. Thus, we inferthat the mafic dykes in southern Qiangtang were emplaced within23Ma between279Ma and302Ma, having the feature of rapid emplacement and extrusion.The basic rocks in southern Qiangtang exhibit the following characteristics:(1)they resemble alkaline rocks which are identified as the products of mantle plume(Whitehead and Luther1975; Morgan1981; Campbell and Griffiths1990; Griffithsand Campbell1990);(2) they are rich in Ti resembling the Emeishan basalts androcks from typical mantle plumes;(3) they are Fe-rich, which may also suggestassociation with mantle plume activity;(4) they are LREE-enriched without a Euanomaly and similar to the oceanic island basalt in composition of trace elements butdepleted in Nb and Ta. The mean ratios of the incompatible elements in the basicrocks in southern Qiangtang are close to those of the Emeishan basalts and OIBbasalts, which suggests a similar origin. Basic-ultrabasic rocks in southernQiangtang are likely to be the products of a mantle plume and the zircon Hf isotopecomposition is similar to that of depleted mantle, which implies that the mafic dykesmay have originated from a depleted mantle source.The three different distribution regions of the mafic dykes may indicate thecombined effect of plate motions and mantle plume activity. The southern Qiangtangblock shifted westwards together with the north–southwards extension as the mantleplume rose and died away300–280Ma ago. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Paleo-Tethys Ocean.