岩浆分异过程的证据:锆石微量元素--以湘南三叠纪王仙岭花岗岩体研究为例

Trace elements in zircon: constrain on magmatic differentiation--A case study on Triassic Wangxianling granitoid in southern Hunan Province

本文以锆石微量元素为着眼点,结合野外勘查和锆石U-Pb测年及Nd-Hf同位素分析,精细研究南岭地区三叠纪王仙岭复式岩体的分离结晶过程。岩体主体部分的粗粒电气石白云母花岗岩(237.8±3.5 Ma)、二云母花岗岩(236.6±5.0 Ma)、强蚀变电气石花岗岩(238.5±3.5 Ma)侵位时代一致,同属于晚三叠世岩浆活动的产物;对应的εHf(t)值分别为-11.8^-7.8、-17.8^-7.4、-16.1^-6.3,εNd(t)值分别为-9.9^-11.1、-10.8^-11.1、-9.2^-10.8;相近的εHf(t)和εNd(t)值说明这些岩浆岩来自同一源区的同一期岩浆事件。三叠纪花岗岩中电气石白云母花岗岩锆石Ce/Sm和Yb/Gd值同时增加的变化趋势以及低的P和Y含量指示存在磷灰石等富MREE和P、Y矿物的分离结晶。另一方面,二云母花岗岩相对亏损LREE和Th,指示锆石结晶之前熔体经历了褐帘石/独居石的分离结晶,而较高的P含量进一步确定了是褐帘石而非富P相独居石的分离结晶;其Nb、Ta含量低、Eu负异常更显著,说明受到金红石和斜长石的分离结晶的影响。电气石白云母花岗岩和二云母花岗岩虽然同期同源,但锆石微量元素变化精细的刻画出岩浆分离结晶过程的差别。本研究表明岩浆岩副矿物地球化学分析是揭示岩浆结晶过程的有效手段。

Objectives: This study targets on trace element in zircon as a breach to elaborate the petrogenesis and differentiation features of the Wangxianling granitoid in southern Hunan Province,Nanling Mountains,on the basisof field work and zircon U-Pb geochronology and Hf isotopes.Methods: The microscope observation of rock sample was conducted based on field work with subsequent zircon U-Pb dating,trace element measurement and Nd-Hf isotopes analysis.Results: The zircon U-Pb dating yielded the age of 237. 8±3. 5 Ma for coarse grained tourmaline-muscovite granite. The two-mica granite emplaced at 236. 6±5 Ma and altered tourmaline granite age was constrained at 238. 5±3. 5 Ma. And their εHf( t) values are confined as-11. 8 ^-7. 8、-17. 8 ^-7. 4、-16. 1 ^-6. 3,and εNd( t)values of-9. 9 ^-11. 1、-10. 8 ^-11. 1、-9. 2 ^-10. 8 respectively. Zircon trace element of the tourmaline-muscovite granite shows the REE reaches 1392 ×10-6~ 2184 ×10-6 with negative Eu anomaly( Eu/Eu*= 0. 10 ~0. 18) and significant positive Ce anomaly( Ce/Ce*= 6. 64 ~ 68. 79). Besides,it contain high P and Y contents ranging from 735 × 10-6~ 1390 × 10-6 and 1931 × 10-6~ 3024 × 10-6 respectively. The Ta and Th contents are confined between 8. 4×10-6~ 21. 7×10-6 and 461×10-6~ 2137×10-6 respectively. In comparison,two mica granite show lower LREE and more prominent negative Eu anomaly( Eu/Eu*= 0. 03 ~ 0. 11) and weaker Ce anomaly( Ce/Ce*= 1. 54 ~ 7. 75). The Y content is higher( 3470 ×10-6~ 4698 ×10-6) and P content( 1777 ×10-6~ 2653 ×10-6) are higher and Ta( 6. 20×10-6~ 17. 26×10-6) and Th( 133×10-6~ 360×10-6) contents are lower.Conclusions: The Wangxianling granitoid is compounded with Late Triassic intrusions, namely the tourmaline-muscovite granite,the two-mica granite and altered tourmaline granite. Their εHf( t) and εNd( t)values overlap with each other and they are product of a same magmatic event. Zircon trace element shows the increase of Ce/Sm-Yb/Gb ratios and low Y and P content of the tourmaline-muscovite granite which suggests there is MREE and Y-bearing mineral such as apatite fractionation. On the other hand,two-mica granite was depleted of LREE and Th relative to tourmaline-muscovite granite which indicates its melt experienced LREE and Th-enriched minerals such as allanite and monazite saturation and the high P content further imply that it’s allanite instead of P-enriched monazite. The low Nb,Ta content and more prominent negative Eu anomaly indicate rutile and feldspar fractionation. The tourmaline-muscovite granite and two-mica granite are contemporary and originated from the same source region,yet zircon trace element tells the distinct crystallization processes. This study suggests accessory mineral chemistry could effectively reveal differentiation and crystallization sequences of its host magma.