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桂东北越城岭岩体是一个由加里东期-印支期花岗岩组成的复式岩基,是目前南岭地区钨锡矿产调查评价的重点对象。本文选取该复式岩体中的加里东期不同岩性花岗岩,包括细粒花岗闪长岩、(粗)中粒斑状二长花岗岩和(中)细粒(斑状–含斑)二长花岗岩为研究对象,进行LA-ICP-MS锆石U-Pb年代学、矿物学、地球化学和Nd-Hf同位素组成研究。研究结果表明,细粒花岗闪长岩和(粗)中粒斑状二长花岗岩具有较低Si O2(<70%)含量和A/CNK值(0.99~1.05),较高Ca O、Ti O2含量和Fe O+Mg O值,中等(La/Yb)N值以及中度亏损Ba、Sr、Eu元素等特征,副矿物以榍石为主加少量磁铁矿,属于I型花岗岩,形成时间为435~438 Ma;(中)细粒(斑状-含斑)二长花岗岩具有富硅(>70%)、富碱、贫钙,高A/CNK值(1.04~1.14),低Fe O+Mg O值和(La/Yb)N值,中到重度亏损Ba、Sr、Eu元素等特征,副矿物含量低,以钛铁矿、独居石为主,为S型花岗岩,形成时间为423~429 Ma,略晚于前者。C/MF-A/MF图解反映I型花岗岩由变质中基性火成岩部分熔融形成,源区具有负且稳定的εNd(t)值(–7.1~–7.9)和εHf(t)值(–6.4~–7.8),平均地壳存留年龄为1.8 Ga左右。S型花岗岩的εNd(t)值(–7.9~–8.8)和tNd2DM值(1.81~1.88 Ga)与I型花岗岩类似,但是其Ca O/Na2O值(0.28~0.64)和相对分散的εHf(t)值(–2.6~–7.9)和tHf2DM值(1.57~1.90 Ga)说明源区可能存在变杂砂岩、变泥质岩和年轻地壳组分的三元混合,且以前两者为主。综合分析华南加里东造山带构造演化序列可以得知,造山带从褶皱缩短、逆冲加厚阶段向伸展垮塌阶段转化而形成的等温降压过程,是形成越城岭花岗岩的诱因。等温降压过程可以通过岩基旁侧新宁-资源深大断裂的松弛调整来实现。
The Yuechengling pluton in the northeast of Guangxi Province is a complex rock foundation composed of Caledonian-Indosinian granite. It is the key target of investigation and evaluation of tungsten-tin deposits in Nanling area. In this paper, the granitic rocks of the Caledonian lithology in the complex rock mass are selected, including fine-grained granodiorite, (coarse) medium-plagioclase granite and (middle) fine-grained The object of study is to study zircon U-Pb geochronology, mineralogy, geochemistry and Nd-Hf isotopic composition of LA-ICP-MS. The results show that the fine granodiorite and the coarse coarse-grained monzogranite have lower contents of Si O2 (<70%) and A / CNK (0.99-1.05), higher contents of CaO, Ti O2 Content and Fe O + Mg O value, medium (La / Yb) N value and moderate depletion of Ba, Sr and Eu. The accessory minerals mainly consist of ores with a small amount of magnetite and belong to type I granite, forming time (435 ~ 438 Ma). The fine-grained (macular-plagioclase) monzogranites have rich silicon (> 70%), rich in alkali and poor in calcium, high in A / CNK (1.04-1.14) Mg O value and (La / Yb) N value, moderate to severe depletion of Ba, Sr, Eu and other characteristics of the secondary mineral content is low, mainly ilmenite, monazite, S-type granites, the formation time of 423 ~ 429 Ma, slightly later than the former. The C / MF-A / MF diagram shows that the type I granites are partially melted by the metamorphic basement igneous rocks with negative and stable εNd (t) values (-7.1 -7.9) and εHf (t) values (-6.4 ~ -7.8), and the average crustal age is about 1.8 Ga. The εNd (t) values (-7.9 ~ -8.8) and tNd2DM values (1.81 ~ 1.88 Ga) of S-type granites are similar to those of I-type granites, but the CaO / Na2O values of 0.28-0.64 and the relatively dispersed εHf (t ) Values (-2.6 ~ -7.9) and tHf2DM values (1.57 ~ 1.90 Ga) indicate that there may be ternary mixed terrigenous alteration of metamorphic rocks, metamorphic rocks and young crust in the source region, and the former two are dominant. Based on a comprehensive analysis of tectonic evolution sequences of the Caledonian orogenic belt in South China, it can be concluded that the isogenic pressure-relief process resulted from the transformation of the fold from shortening to thickening and from thrusting and thickening to collapsing and collapsing are the inducing factors for forming the Yuechengling granite. The isothermal buckling process can be achieved through the slack adjustment of the Xinning-deep resource fault beside the rock foundation.