闽西南龙岩地区印支期天宫山花岗岩体成因

Petrogenesis of Tiangongshan Indosinian Granites in Longyan Area,Southwest Fujian

天宫山花岗岩体成因的研究对闽西南地区岩浆演化及动力学过程有重要意义.天宫山岩体岩性主要为正长花岗岩.前人于天宫山岩体中利用K-Ar法测得年龄值为146~149Ma,天宫山正长花岗岩中测得锆石LA-ICP-MSU-Pb年龄为233±2.0Ma、230±2.8Ma,为晚三叠世,属印支期.该岩体富硅,富碱,σ=1.21~2.55,A/CNK=0.97~1.73,属钙碱性系列,准铝质到过铝质范畴.岩石ΣREE较高,LREE相对富集,贫Al2O3和Sr,富Y和Yb,发育有明显的显微文象结构,具有较强的铕负异常,中等铈负异常到弱正异常;亏损大离子亲石元素,富集高场强元素.Ga/Al值高,具有A型花岗岩特征.w(P2O5)平均值为0.02%,低于高分异S型花岗岩;w(Na2O)平均值为2.93%,高于高分异S型花岗岩;全铁含量w(TFeO)平均值为1.15%,高于高分异I型花岗岩.锆石饱和温度平均值为729.8℃.εHf(t)全部为负值(-5.29^-10.69),表明其物质起源可能主要为古元古代下地壳物质.参与成岩作用的岩浆来源于地壳物质的部分熔融,为同碰撞环境下形成的壳源型铝质A型花岗岩.在印支期碰撞-挤压为主的造山运动背景下,岩体在高温环境中经历了古元古代下地壳物质的初步熔融,经印支期运动伸展,部分地幔物质参与了经底侵作用.

The research of petrogenesis about Tiangongshan Indosinian granites have great significance on the magmatic evolution and dynamic process in Southwest Fujian. The Tiangongshan pluton is dominated by syenogranite. Previous studies obtained the K-Ar age of 146-149 Ma from its granites, while U-Pb zircon analyses yielded consistent age of 233±2.0 Ma and 230±2.8 Ma by LA-ICP-MS in our study, indicating that the pluton was formed in Late Triassic and Indosinian tectonic movement. The granites are characterized by high silicon and alkali (σ=1.21-2.55,A/CNK=0.97-1.73), belonging to quasi aluminum to peraluminous calc-alkaline series. The granites have high ΣREE and relative accumulation of LREE, and are deficient in Al2O3 and Sr, and rich in Y and Yb, showing obvious micrographic texture with relatively high negative Eu anomaly and moderately negative to weak positive Ce anomaly;the granites are depleted in LILE and enriched in HFSE.The Tiangongshan pluton has the characteristics of A-type granite with high Ga/Al value. Its w (P2O5) averages on 0.02%, lower than that of highly fractionated S-type granite, w(Na2O) on average of 2.93%, higher than that of highly fractionated S-type granite, and w(TFeO) on average of 1.15%, higher than that of highly fractionated I-type granite.The average saturation temperature of zircon is 729.8 ℃.The εHf(t)all is negative (-5.29 to -10.69), suggesting possible material sources from the lower crustal material of Paleoproterozoic. The magma of the diagenetic stage, derived from partial melting of the crustal material, is crustderived aluminous A-type granite formed in the syn-collisional settings. Through the stage of Indosinian collisional compressional orogeny, the Paleoproterozoic lower crustal material of the pluton initially melted. Stretched by Indosinian tectonic movements, part of the mantle material was involved in the underplating.