Gneiss

Classification：Metamorphic Rock

It is a metamorphic rock with coarse-grained metamorphic rocks with gneiss structure or banded structure. It is formed in deep crust beneath the alpine area with complex chemical composition.

Gneiss (gneiss) is a rock formed by deep metamorphism of magmatic rocks or sedimentary rocks. The structure of the dark and light minerals is characterized by a directed or strip shaped gneiss structure, with a polycrystalline structure. The main minerals are quartz, feldspar, hornblende, mica and so on. According to the original rock, it is divided into "gneiss" metamorphosed by magmatic rocks and "accessory gneiss" metamorphosed by sedimentary rocks. The compressive strength of general rock blocks is 117.7 to 196.1 MPa, and the compressive strength of rock blocks decreases when mica content is large. The shear strength along the direction of foliation and smaller resistance.

A metamorphic rock of gneiss. It is usually the product of middle - Advanced metamorphism. It has obvious flax like structure. It is mainly composed of feldspar, quartz and various dark minerals (mica, hornblende, pyroxene and so on). According to the material composition of rock, it can be divided into aluminum rich gneiss, plagioclase gneiss, alkali long (two long) gneiss and calcareous gneiss. Gneisses are mainly composed of feldspar and quartz, medium coarse grained texture and metamorphic rocks of gneisses or banded structures.

Main causes

The gneiss mainly consists of three types: the black cloud diagonal gneiss, the two long granite gneiss and the A type granitic gneiss. The geochemical discriminant results show that their original rocks are all igneous rocks. Geochemical characteristics of different gneisses are different, indicating that their original rocks have different formation and evolution history.

Biotite plagioclase:
The black cloud diagonal gneiss is characterized by negative Eu anomaly, even obvious positive anomaly (delta Eu=0.60 ~ 1.45), strong Ba anomaly and no obvious Sr anomaly. Its geochemical characteristics are similar to the acidic endmembers of Neoproterozoic Shuangfeng volcanic rocks on the northern margin of Yangtze craton.

Two long granite:
Similar to black cloud plagioclase gneiss, the degree of fractionation between light and heavy rare earth elements in two granitic gneiss is also strong (La_N/Yb_N=6.3 to 17.2).
But it is characterized by strong negative Eu anomalies (delta Eu=0.27 to 0.54), strong Sr negative anomalies and weak Ba negative anomalies. You are obviously different from biotite gneiss.

A type of granite:
The REE pattern and spider web diagram of A type granite gneiss are similar to those of two long granitic gneiss. The difference is Nb and Ta are relatively weak. E _ several types of gneiss area in (Nd) (T) have little difference, reflect the genetic relationship between them may be some original rock.

Generality and difference:
Their Nd isotopic age T_ (DM) is concentrated around 2.0Ga, indicating that their source regions are mainly early Proterozoic crustal materials, and their lithology is homogeneous. It is conjectured that the original rocks of the two long granite gneiss and the black cloud diagonal gneiss are homologous, while the A type granite is formed by partial melting of the crustal material left by the dehydrated perilla pyroxene after the formation of the large-scale granitic magma.