北秦岭晚三叠世关山岩体地球化学特征 及成因机制.

Guanshan area is located in the junction of North Qinling and North Qilian orogenic belt, and it has a lot of controversy about the genesis of Guanshan pluton at present. The zircon U-Pb dating results show that the monzogranite of Guanshan pluton is formed in(236.3±4.0)Ma(the number of sample is 6, MSWD is 2.6). Late Triassic Guanshan monzogranite in North Qinling displays the characteristics of high Si(the content of SiO2 is more than 69%)and alkali-rich(the content of K2O is more than 3.84%, that of Na2O is more than 4.18%), Mg# is on the high side(50.4-51.0), LREE is enriched, HREE is depleted and flat, Eu has a non-obvious negative anomaly(0.75-0.82). The rock is enriched in Sr, Rb, Ba, and depleted in Y, Yb and other elements, the ratio of Sr/Y is 37.5-60.7, indicating that the rock has the propertieswith adakite. Zircon Lu-Hf isotope displays that 5 grains of the monzogranite have negative εHf(t)(-21.59--4.49)with corresponding two-stage Hf pattern ages of 1 538-2 622 Ma, and 1 grain has positive εHf(t)(0.19)with two-stage Hf pattern age of 1 245 Ma, indicating that Guanshan pluton is derived from the melting of Neoproterzoic crustal material. The rock has high SiO2 and Al2O3 contents, low MgO, Cr and Ni contents, indicating the pluton derives from the melting of crustal material. Compared with the standard value of the crust, the higher Mg# indicates the addition of the mantle component in the source of Guanshan pluton. The petrography, geochemistry, isotopic characteristics and regional geological data comprehensively show that the monzogranite of Guanshan pluton is formed under the tectonic setting of collisional compression, and the lower crust is thickened, dehydrated and melted with the addition of mantle components. [ABSTRACT FROM AUTHOR]