华北克拉通北缘晚古生代尚义钾质花岗岩的成因分析:来自岩石地球化学的证据

产于华北克拉通北缘中段的晚古生代尚义钾质花岗岩体规模很小,岩性为钾长石斑晶(～40%)的黑云母钾长-二长花岗岩,矿物组合为钾长石(50%～60%)、斜长石(10%～20%)、石英(20～30%)、黑云母(～5%)及少量绿帘石、单斜辉石和磁铁矿。SiO2含量为65.69%～70.90%,K2O+Na2O为7.22%～9.14%,K2O/Na2O比值为0.85～1.75,Al2O3为13.58～15.09%,A/CNK值为0.89～0.93,Mg#为43～46,稀土含量中等,轻重稀土中等分异,轻微负Eu异常,富集LILE和地幔元素Cr,Nb、Ta、Ti负异常,表现为富钾、偏铝质、钙碱性的I型花岗岩。通过野外观察及岩石地球化学分析可以确定钾质花岗岩是幔源基性岩浆分离结晶并同化混染早期奥长花岗岩围岩形成的。钾质花岗岩位于尚义-赤城断裂带中,其发育的构造背景应为晚古生代时古亚洲洋板块向华北克拉通俯冲形成的活动大陆边缘。

Genesis of Late Paleozoic Shangyi potassic granite in the northern margin of the North China Craton: Petrochemistry evidence

The Shangyi potassic granite body in the central section of the northern margin of the North China Craton is small. It consists of biotite-bearing moyite and monzogranite with K-feldspar phenocrysts（~40%）. It consists mainly of K-feldspar （50%-60%）, plagioclase （10%-20%）, quartz （20%-30%）, biotite （~5%） and little epidote, clinopyxene, magnetite. The geochemical compositions are： SiO2 65.69%-70.90%, K2O＋Na2O 7.22%-9.14%, K2O/Na2O 0.85-1.75, Al2O3 13.58%-15.09%, A/CNK 0.89-0.93, Mg# 43-46. It is enriched in LILE and Cr, with moderately fractionated REE patterns and slight negative Eu anomalies, and negative Nb, Ta, Ti anomalies. Shangyi potassic granite is potassium-rich, metalulminous, I-type calc-alkaline granite. After field observation and petrochemistry analyses, we propose that the Shangyi potassic granite derived from fractional crystallization of basic magma of mantle origin, accompanied by assimilation of trondhjemite wallrock during emplacement. This potassic granite is located in the Shangyi-Chicheng fault zone, and its tectonic setting was active continental margin formed by subduction of the Paleo-Asian ocean plate beneath the North China Craton during Late Paleozoic.