Abstract:

The Shadegai granite is located in Hadamengou large gold-molybdemum orefield, in the north margin of North China craton. The SHRIMP U-Pb weighted average age of 15 zircon grains from the Shadegai granites is (221.6±2.1) Ma（MSWD=1.6），which indicates the granite emplaced in the medium Indosinian epoch. Geochemical characteristics show that the granite is characterized by high silicon(SiO2=71.21%Northern margin of North China craton； Shadegai granite； geochronology； geochemistry； Triassic 73.67%), potassium-rich (K₂O/Na₂O=1.01-1.37), rich alkaline(K₂O+Na₂O=8.23%-9.96%), weakly peraluminous（Al₂O₃=13.11%-14.31%）. The Rittmann index (σ) of granite varies from 2.43 to 3.52, which means that the rock samples are calc-alkaline to alkaline. The ∑REE varies from 185.70×10^-6 to 96×10^-6, the ratios of LREE/HREE and La_N/Yb_N are 17.68-14.92 and 22.85-16.58, respectively, and δEu is weakly negative anomalies. Trace elements composition exhibit such characteristics of A-type granite, as depletion of Nb, Ta, P, Ti, Sr，and enrichment of K, La, Ce, Hf. The values of (⁸⁷Sr/⁸⁶Sr)i is from 0.705 31 to 0.702 29， (¹⁴³Nd/¹⁴⁴Nd)i from 0.511 682 to 0.511 620，and ε_Nd(t) from -13.1 to -14.3, indicating that the magma source of Shadegai granite should have been mainly of crust-derivation. Nd isotope two-stage model ages(T_2DM) are 2 061-2 160 Ma. In the tectonic model diagram of lead isotopic compositions, the granite samples are plotted between the mantle and lower crust curves. The comprehensive analysis suggests that the Shadegai granite formed under the tectonic setting of transformation from syn-collision to post-collision. Statistics indicate that the magmatism and mineralization took place extensively in the northern margin of North China craton during the Indosinian epoch.

Key words: Northern margin of North China craton, Shadegai granite, geochronology, geochemistry, Triassic