诸广山南体桃金洞花岗岩成因和铀成矿潜力探讨

摘要

In this paper, geochronology and petrogeochemistry and Sr-Nd isotopic geochemistry of Taojindong granite are studied and compared with those of other Indosinian granites in the eastern part of the southern Zhuguangshan composite pluton. LA-ICP-MS zircon U-Pb dating for Taojindong granite yields an age of 204 ± 2.1 Ma, suggesting its intrusion in the late Indosinian period. Chemically, this granite is peraluminou, and shows relatively high contents of silica(SiO2=69.7%~75.0%), alkali(K2O+Na2O=7.74%~9.08%)and FeOT, and low abundance of magnesium. Thus, the Taojindong granite is characterized as calc-alkalic- to alkali-calcic, peraluminous, and ferroan. The rock samples display high total REE contents(∑REE=226~272 × 10-6), with LREE/HREE ratios of 6.27~11.4 and(La/Yb)N ratios of 4.01~15.0, and a negative Eu anomaly(δEu=0.15~0.42). This granite is enriched in Rb, Th and U,and depleted in Ba, Sr, Ti and Eu, showing characteristics of a typical low Ba and Sr granite. Isotope geochemical analysis reveals high and variable initial 87Sr/86Sr ratios(0.71922~0.72040), yet low and constantεNd(t)values(-10.0~-10.2)with two stage model age of 1.80~1.82 Ga. All these features suggest that the Taojindong granite is a typical crust-derived granite. The granitic magma may have been derived from low-intermediate degree partial melting in a crustal extension-thinning tectonic environment in the late Indosinian period, of metagreywackes rocks whose origin was crust rocks separated from the mantle in the Paleoproterozoic. In the southern Zhuguangshan composite pluton, compared with non-uranium-mineralization-bearing granites, the uranium-mineralization- bearing granites experienced stronger hydrothermal alterations, and show wider variations in FeOT/(FeOT+MgO) ratios, higher uranium concentration, more depletion in CaO, Ba, Sr, Ti and Eu, lower εNd(t) values and more ancient two stage model ages. The non-uranium-mineralization-bearing granites were derived from metagreywackes and relatively depleted in U. Taojindong granites have not experienced significant hydrothermal alteration and have no characteristics of intense alteration like uranium-mineralization-bearing granites. The geochemical features of the Taojindong granites are similar to those of the Indosinian non-uranium-mineralization-bearing granites in the southern part of the Zhuguangshan composite pluton, and its uranium potential is slight.%文中对位于湘赣粤三省交界处的诸广山南体桃金洞花岗岩进行了锆石U-Pb年代学和岩石地球化学的研究,并将其与诸广山南体东部其他印支期非产铀和产铀花岗岩进行了对比.LA-ICP-MS锆石U-Pb年龄为204±2.1 Ma,为印支晚期岩浆活动的产物.岩石地球化学组成呈过铝质,硅和碱含量偏高(SiO2=69.7%~75.0%,K2O+Na2O=7.74%~9.08%),富铁、贫镁,属于碱钙性/钙碱性-过铝质-铁质花岗岩.稀土元素总量较高(∑REE=226~272×10-6),LREE富集(LREE/HREE=6.27~11.4,(La/Yb)N=4.01~15.0),Eu亏损较明显(δEu=0.15~0.42),富集Rb、Th和U,亏损Ba、Sr、Ti和Eu,属于典型的低Ba、Sr花岗岩;(87Sr/86Sr)i值较高(0.71922~0.72040),εNd(t)值较低(-10.0~-10.2),两阶段Nd模式年龄为1.80~1.82 Ga.上述特征表明,桃金洞花岗岩属于典型的壳源型花岗岩,是在地壳伸展-减薄构造背景下,由古元古代地壳岩石演变而成的变质杂砂岩组分岩石经中低程度部分熔融形成.对比研究显示,诸广山南体印支期产铀花岗岩蚀变作用强,FeOT/(FeOT+MgO)比值变化较大,CaO含量低,主要为碱钙性花岗岩,Ba、Sr、Ti和Eu亏损更强烈,εNd(t)值更低和Nd模式年龄更古老.非产铀花岗岩源岩以砂质岩为主,U含量相对较低.桃金洞花岗岩未经后期明显热液蚀变作用,不具有产铀花岗岩蚀变强烈的特点,地球化学特征相似于诸广山南体印支期非产铀花岗岩,铀成矿潜力可能不大.