黔西赫章2.5 Ga石英二长闪长岩的发现及其地质意义

黔西位于扬子板块西南缘，有关扬子板块内约2.5 Ga的岩浆事件记录极少。最近，对黔西赫章发现的石英二长闪长岩的研究表明，其成岩年龄为（2 542±9）Ma，为该区首次发现的新太古代晚期岩石。该岩石主要由斜长石、碱性长石、石英、角闪石、黑云母等矿物组成，具有较高的SiO₂（59.2%~60.6%）、Na₂O（4.70%~4.85%）质量分数和Na₂O/K₂O（3.02%~3.19%）比值，以及较低的Al₂O₃（17.0%~17.7%）、MgO（2.42%~2.61%）质量分数。样品稀土元素总质量分数为120×10-6~221×10-6，（La/Yb）_N=8.32~17.98，轻、重稀土元素分异不明显，表现为弱的右倾稀土元素配分模式，并具有弱的Eu负异常。在微量元素蛛网图上，样品富集大离子亲石元素（Ba、Sr）、亏损高场强元素（Nb、Ta、Ti），具有弱负到正的ε_Hf（t）值（-0.51~+6.93）和正的ε_Nd（t）值（+0.22~+0.88），与太古宙低铝型高重稀土元素TTG的地球化学性质基本一致。综合研究显示，该岩石可能是在角闪岩相下，下地壳基性岩部分熔融形成的，源区残留相主要为金红石以及斜长石。黔西约2.5 Ga岩石的发现证实了扬子板块西南缘具有新太古代基底的存在，这为扬子周缘新太古代岩浆成因的碎屑锆石研究提供了依据，为扬子板块新太古代地壳演化提供了新的信息。 

Discovery of 2.5 Ga quartz monzodiorite and its geological significance in Hezhang,western Guizhou

The western Guizhou is located at the southwest margin of the Yangtze Block.The record of 2.5 Ga magmatic events in the Yangtze Block is very little.The Hezhang quartz monzodiorite recently discovered in western Guizhou shows that the LA-ICP-MS U-Pb age of the rock is (2 542±9)Ma, which is the first late Neoarchean rock found in this area.The rock is mainly composed of plagioclase, alkaline feldspar, quartz, amphibole, biotite, etc.It has medium high content of SiO₂ (59.2%-60.6%), Na₂O (4.70%-4.85%), and Na₂O/K₂O (3.02%-3.19%)ratios, and low content of Al₂O₃ (17%-17.7%) and MgO(2.42%-2.61%).The total number of rare earth elements (REE) was 120×10-6-221×10-6, (La/Yb)_N=(8.32-17.98), and the differentiation of light- and heavy-REEs was not obvious, which showed a right-leaning type.The rocks have negative anomaly of Eu, enrichment in large ion lithophile elements (LILEs)(Ba, Sr) and deficency in high field strong elements (HSFEs) (Nb, Ta, Ti).The samples have weak negative to positive ε_Hf(t) value (-0.51-+6.93) and positive ε_Nd(t) value (+0.22-+0.88).The rock has the characteristics of low-Al type and high HREE TTG series.The comprehensive study shows that this rock may be formed by partial melting of metasomatism between the oceanic subducted plate and the water-bearing mantle wedge in the low-pressure shallow environment under eclogite facies.The residual phases in the source area are mainly rutile and a small amount of amphibole.This first report of 2.5 Ga rocks confirms the existence of the Neoarchean basement in the southwest margin of Yangtze Block, which provides a basis for the study of detrital zircons of Neoarchean magmatic origin around the Yangtze Block and provides new information for the evolution of the Neoarchean crust of the Yangtze Block.