皖南蛇绿岩伏川铬铁矿床成因:铬铁矿母岩浆证据

皖南蛇绿岩带位于江南造山带北东段，该区地幔橄榄岩多已发生蚀变甚至全部蛇纹石化，成为制约该区深部研究工作的一个现实瓶颈.为查明该蛇绿岩及赋存其中的伏川铬铁矿床的成因，本研究通过电子显微镜以及电子探针等手段，对皖南蛇绿岩和不同类型铬铁矿石（Ⅰ类以铬绿泥石为主要脉石矿物和Ⅱ类以异剥辉石为主要脉石矿物）中铬铁矿及其共生矿物进行深入研究，发现铬铁矿为典型的阿尔卑斯型高铝型铬铁矿，大多发育裂隙结构，在颗粒边缘或裂隙处部分蚀变为铁铬铁矿.对铬铁矿核部未蚀变部分进行研究，得出方辉橄榄岩中铬尖晶石Cr#（100×Cr/（Cr+Al））为54.12~65.18，Mg#（100×Mg/（Mg+Fe2+））为42.37~54.84，铬铁矿石中铬铁矿Cr#为53.97~62.29，Mg#为59.49~68.57.铬铁矿母岩浆成分和MORB（mid-ocean ridge basalt）基本一致，表明伏川铬铁矿结晶时母岩浆的成分可能为MORB.对其氧化环境的研究发现其成岩成矿环境为低氧逸度（-0.14至+0.68log（QFM）），并具有从MORB向SSZ（suprasubduction zone）过渡的特征.结合野外地质现象，认为铬铁矿石可能是地幔橄榄岩和洋中脊玄武岩浆反应而形成，而方辉橄榄岩又体现出SSZ特征，说明皖南蛇绿岩应该是地幔橄榄岩部分熔融、岩石-熔体反应和板块俯冲等综合作用的结果，这为研究江南造山带的构造和演化提供重要依据. 

Genesis of Fuchuan Chromitites at South Anhui,Implications from the Parental Melts

South Anhui ophiolite, one of the oldest ophiolites in southern China, is located in the northeastern most segment of Jiangnan orogen. The ultramafic rocks in this area are extensively serpentinized which constrains the researches here and it is challenging for further studies. In order to investigate the origin of the South Anhui ophiolites and the Fuchuan chromitites, detailed observation with microscope and electron probe micro-analyzer was made for the mineralogy analysis of chromites and paragenetic minerals in harzburgites and chromitites. The chromites can be divided into two types, namely, type Ⅰ with k?mmererite as the gangue mineral, and type Ⅱ with diallage as the gangue mineral. All the chromites are typical Alpine-type high aluminum chromites, most of which develop fractures, and along the rim and fractures are altered into ferritchromite. The unaltered core part of the chromites which retains the pristine composition was studied, and the spinels in harzburgites are characterized by moderate Cr# (100×Cr/(Cr+Al), 54.12-65.18) and low Mg# (100×Mg/(Mg+Fe2+), 42.37-54.84), and those of the chromites from the chromitites are 53.97-62.29 and 59.49-68.57, respectively. The calculated component of parental melts of the chromitites is similar to MORB (mid-ocean ridge basalt), indicating the derivation from MORB. The oxygen fugacity is low (-0.14 to +0.68log(QFM)), showing the feature of transition from MORB to SSZ (suprasubduction zone) setting. Combining with the field phenomena, it is believed that chromitites might be formed from the interaction of peridotites and MORB, and together with the SSZ feature of the harzburgites, reflecting that the formation of South Anhui ophiolites might be the result of the combined effect of partial melting of the peridotites, rock-melt interaction, and plate subduction, which provides vital evidence for the tectonic and evolution of the Jiangnan orogen.