The Geochemical Study On The Mafic-ultramafic Subvolcanic In Heqing, Western Yunnan

This study reports the data of bulk-rock major element, trace element, Sr-Ndisotopes of the Cenozoic basic-ultrabasic rocks from Runanshao, Heqing region, anddisscusses the causes and deep processes of the magma. The Cenozoicbasic-ultrabasic rocks are located in the Jinsha River-Ailaoshan-Red Riverfaultzone in western Yunnan, Southeast of the Qinghai-Tibet Plateau. These Cenozoicbasic-ultrabasic rocks were emplaced at around40Ma. They are characterized by highMgO and low K2O contents. According to the petrography, these samples are picriticrocks (picritic porphyrites and picritic cumulates). They comprise olivine,clinopyroxene, plagioclase and a little spinel. Among them, most olivine crystals areperidot crystals (Mg#:82.85～88.82), most spinel crystals are chrome spinel crystals(Cr#:69.27～71.32). The bulk-rock composition is characterized by low SiO2(37.88wt%～44.86wt%) and high MgO and TiO2(8.83%～30.94wt%and1.327%～3.332%wt%). The standardized graphical of trace amounts and rare earth elements ofpicritic porphyrite is similar to OIB, which is rich in LILE elements (especially Rb,Ba, Th and U) and Light rare earth elements. The Sr-Nd isotopes showed that thebulk-rock has relatively high initial value of87Sr/86Sr (0.70491～0.70713) and lowinitial value of143Nd/144Nd (0.512507～0.512639). Low ratio of La/Ta and La/Sm(14.5～15.79and3.79～4.16) in these picritic rocks explained the samples has littleeffect from crustal contamination. All samples are not depleted in HFSE and Nb/La＞1, showed that these samples has little effect from crustal contamination andfractional crystallization. According to the balance estimates of Mg-Fe in olivine andmelt, the digital of MgO and FeO of resumptive primary magma are about13%and10%, and the primary magma is picritic magma. By using ultrabasic rocks’temperature-pressure model, the estimated temperature and pressure is1450℃and2.6Gpa separately. Corresponding to the pressure, the depth is about87km, which wecan speculate that the magma may be originated in the mantle source where garnet isrelatively stable. Zr/Nb’s ratio of the picritic rocks is5.49～6.82(＜10), and Nb/La’sratio is1.02～1.10(＞1), which both are similar with the characteristic of OIB. In theεNd（t）-（87Sr/86Sr）t FIG., five sixth of the samples fall into the OIB region, which means the samples are OIB-type magmatic rocks. We can speculate that the mafic-ultramafic subvolcanic magma in this aera originated in the asthenosphere in thedepth of87km, and they are the products of low degree of partial melting in hightemperature and high pressure. The forming time of the mafic-ultramaficsubvolcanic magma in this aera is consistent with the sub-vocanic rocks fromDazi-Jiama and the Gaoligong mafic dykes, and they are all the products of slabbreak-off. After the collision of Indian and Eurasian plates, asthenosphere upwellingand partial melting in the course of the break-off of subducting Tethys slab and Indiancontinental plate, cause the formation of the picritic OIB-type magmatic with a smallscale in long and narrow distribution.