Abstract:

In this paper, a series of analyses including field observation, LA-ICP-MS zircon U-Pb isotopic dating, major element and trace element were performed on the granites in Early Carboniferous in Maodeng of Xilin Hot, Inner Mongolia in order to make the discussion of tectonic environment of the study area. Two types of granites were identified which of granodiorite and monzonite granite are invaded into the former. The result of the LA-ICP-MS zircon U-Pb isotopic dating are (330.6±1.8) Ma for granodiorite and (327.7±2.6) Ma for monzonite granite, which suggests that the granites formed in the Early Carboniferous. The geochemical feature of major elements in the granodiorite is strongly peraluminous (A/CNK=1.32)and of calcium-alkali. The pattern of LREE enrichment type indicates the active continental margin environment with the feature of low ∑REE(67.65×10^-6), (La/Yb)_N (8.37) and slight negative anomaly. The trace element geochemistry shows evident enrichment of LILE(Rb, Ba, Sr), heat producing element(Th, U, K) and HFSE(Zr, Hf) with strong deficit of Nb, P and Ti, which imply that the samples are arc magmatic suites coming from the lower crust with few mantle-derived magma. The geochemical feature of major elements in the monzonite granites is of high SiO₂(75.85%) and K₂O+Na₂O(8.54%), low Al₂O₃ relatively, which belongs to weak peraluminous(A/CNK=1.01) series and high-K calc-alkali series. The feature of REE are divided into two types. One type is of high ∑REE(120.28×10^-6) with negative Eu anomalies(0.88), LREE/HREE (4.96), (La/Yb)_N (10.09), which shows the strong differentiation between the HREE and LREE. The patterns of the other type granites show the slightly differentiation like the “sea gull” with low ∑REE(54.28×10^-6), strong Eu negative anormalies (0.27), LREE/HREE (1.99), (La/Yb)_N (1.50), which comes from the remelting of the low crust.The trace element geochemistry shows evident enrichment of LILE(Rb), heat producing element(Th, U) and HFSE(Zr,Hf) with strong deficit of Sr,Ba,P,Ti,which implies that the sample are mature arc granite coming from the remelting of the low crust magma in the continental marginal environment. Discrimination diagrams display that the granodiorites are I type representing active continental margin environment, but the monzonitic granites are A2 type representing the stretch environment behind the arc of the active continental margin which form the I-A complex. These geochemical granites of different tectonic environments display that the subduction from Paleo-Asian Ocean to the Siberian plate still existed during the Early Carboniferous in Maodeng of Xilin Hot, Inner Mongolia, which indicates that the Paleo-Asian Ocean didn’t close in Early Carboniferous.

Key words: I-A type granite, LA-ICP-MS zircon U-Pb isotopic dating, active continental margin environment, Paleo-Asian Ocean, Xilin Hot