东秦岭中泥盆世成矿海盆中一类特殊岩石—方柱黑云岩

A SPECIAL ROCK-SCAPOLITE BIOTITE ROCK IN THE MIDDLE DEVONIAN PERIOD SEA BASIN IN THE EAST QINLING

赋存层控金属矿床的东秦岭中泥盆统中发育一类特殊岩石——方柱黑云岩。野外和室内研究表明,其为与火山及火山热卤水活动有关的地热异常海盆中的一种特殊沉积相。岩石学、矿物学、岩石化学、矿物化学及稀土元素特征也为其原岩经过卤水“浸泡”蚀变和埋藏热变质形成方柱黑云岩类的成因提供了令人信服的证据。据此,推测出秦岭中泥盆世层控金属矿床形成的地热异常背景。

The metallogonic basin of polymetallic ores in the Shanyang-Zhashui area of Shaanxi is located in the east part of the middle Devonian metalloginic belt of the Qinling Mountain.The stratiform deposits occurred in a suit of deep-stibdeep marine fine clastic sediment sepuence ( weak metamorphosed ) . In the fine clastic sedimentary sequence a special rock with a typical mineral assemblage of scapolite +biotite, scapolite biotite rock, was found. The stratigraphic positions of the scapolite-biotite rockare still stable.It was discovered in the Qingshiya Group and the Chigou Group.In the Qingshyya Group it is more extensive than in the ChigouGroup.Hydrothermal-secimentary polymetallic sulfide beds were produced at the middle and lower parts of the Qingshiya Group above the stratigraphic horizon of the rock group.The existence of the scapolite biotite group has proved an brine's activity with abnormal temperatures at the sea floor during the middle Devonian Epoch. The appearance of scapolite in the rock shows clearly a speciality of the mudstone mineral assemblage. According to the special granular textures and the optical mineralogy (compositional ring texture ) of the scapolite, it can be infered that the primitive materials to form the scapolite are a kind of far-pyroclast.In a given depression place of the sea floor, this kind of elastics was transformed into scapolite through a long time 'soaking effect in a kind of brine. The rock has an unusual high contents of Na2O and Cl-, which are higherl-5 times than those of same kind of mudstone in the whole world.The petrochemical features vary with the increasing scapolite content in the rock, resulting in the ascendant Na2O content in linearity and the regularly decrease in K2Ocontent.The monomineral chemical properties of the scapolite show a good regularity according to what kind of rock it existed in. These reflected the characteristics that the pyroclast existed in the primitively normal sediment mudstones and was reformed by abnormal brine. The REE features of the total rock and the projection results of the chemical parameter calculation further verified the following conclusions: 1) the evidently volcanic activities during the formation of the stratiform deposits of the middle Devonian in the east part of the Qinling Mountain; 2 ) the abnormal brine at the bottom of depression was formed by far-volcanic hot spring.