广西岑溪地区糯垌A型花岗岩的厘定及其构造意义

Confirmation of the Nuodong A-Type Granite in Cenxi City of Guangxi and its Tectonic Significance

广西糯垌岩体位于华夏古陆与钦杭结合带的交接部位,主要岩性为黑云母钾长花岗岩,其锆石LA-ICP-MS U-Pb年龄为(151.1±1.9)Ma和(152.6±1.2)Ma,显示为燕山早期晚侏罗世岩浆活动的产物。糯垌岩体具有富碱、高钾、高铁、贫镁的特征,alk的质量分数为10.29%~10.91%,Fe OT的质量分数为3.60%~5.10%,Fe OT/Mg O值为10.33~11.69。岩石为碱性准铝质系列,δ43为4.78~5.38,A/CNK值为0.96~0.98,A/NK值为1.21~1.24;具有右倾型的稀土配分模式图,稀土总量较高,具强烈的负铕异常,其∑REE值为(610.03~790.48)×10^(-6),δEu值为0.30~0.37。原始地幔标准化蛛网图显示明显亏损Ba、Sr、P、Ti,富集Rb、Th、La、Ce、Zr、Hf、Y等元素;具有较高的10 000Ga/Al值(3.03~3.31)和Zr+Nb+Ce+Y值((857.7~1244.4)×10^(-6))。上述地球化学特征表明糯垌岩体为铝质A型花岗岩,同时具较低的Ce/Nb、Y/Nb、Yb/Ta元素比值,暗示为A1型非造山花岗岩。εNd(t)值为-2.25^-1.52,εSr(t)值为22.34~37.59,TDM值为(0.86~1.24)Ga,表明糯垌岩体来源于中新元古代的地壳物质,可能有地幔物质的参与。糯垌岩体形成于板内环境,说明广西岑溪地区晚侏罗世经历特提斯构造域向太平洋构造域转换后进入板内非造山构造环境。

The Nuodong pluton in Guangxi is located at the connection between the Cathaysia and Qinzhou-Hangzhou junction zone. The main rock type is biotite K-feldspar granite,and the LA-ICP-MS zircon U-Pb age of two zircons is（ 151. 1 ± 1. 9） Ma and（ 152. 6 ± 1. 2） Ma,respectively,indicating that it is the product of Late Jurassic magmatic activity in the early Yanshanian period. This pluton has characteristics of rich alkali,high potassium and iron and poor magnesium（ alk = 10. 29% ~ 10. 91%,FeO T= 3. 60% ~ 5. 10%,FeO T/MgO = 10. 33 ~ 11. 69）. The granitoid is alkaline（ δ43= 4. 78 ~ 5. 38） and metaluminous series（ A/CNK = 0. 96 ~ 0. 98,A/NK = 1. 21 ~ 1. 24）. The rare earth elements have a right-type distribution pattern,with high content of rare earth elements（ ∑REE =（ 610. 03 ~ 790. 48） × 10-6） and strong negative europium anomaly（ δEu = 0. 30 ~ 0. 37）. On the primitive mantle-normalized spider diagrams,the granites show positive Rb,Th,La,Ce,Zr,Hf and Y anomalies,negative Ba,Sr,P and Ti,and high 10000 Ga/Al（ 3. 03 ~ 3. 31） and Zr ＋ Nb ＋ Ce ＋ Y values（（ 857. 7 ~ 1244. 4） × 10-6）. The above geochemical characteristics indicate that the Nuodong pluton is aluminum A-type granite,and its low incompatible element ratios（ Ce/Nb,Y/Nb,Yb/Ta） suggest it is a non-orogenic A1 type granite. The Sr-Nd isotopes（ εNd（ t） =-2. 25 ^-1. 52,εSr（ t） = 22. 34 ~ 37. 59,TDM= 0. 86 ~ 1. 24 Ga） indicate that the Nuodong pluton is derived from the Meso-Neoproterozoic crust material and may be involved in mantle material. The Nuodong pluton was formed in an intraplate environment,which indicates the Cenxi area of Guangxi has undergo the Tethyan tectonic domain converted into the Pacific Ocean tectonic domain in the Late Jurassic and entered a non-orogenic tectonic environment.