What Is a Gneiss?

Gneiss is a rock formed by deep metamorphism of magmatic or sedimentary rocks. It has the characteristic of gneiss-like structure with a directional or band-like intermittent arrangement between dark and light-colored minerals, showing a crystalline structure. The main minerals are quartz, feldspar, amphibole, and mica. According to the difference of the original rock, it is divided into "ortho-gneiss" metamorphic from magmatic rocks and "sub-gneiss" metamorphic from sedimentary rocks. Generally, the compressive strength of rock blocks is 11.7 ~ 196. 1 MPa. When the content of mica is large, the compressive strength of rock blocks decreases. The shear strength is small along the direction of the sheet. ^[1]

Gneiss is made by

Gneiss A metamorphic rock. Usually medium-advanced

Rocks can be roughly divided into three types according to different formation methods:

Gneiss mainly includes three types: helitic oblique gneiss, Erchang granitic gneiss, and A-type granitic gneiss. The geochemical discrimination results show that their original rocks are all igneous rocks. The geochemical characteristics of different types of gneiss are different, reflecting that their original rocks have different formation and evolution histories.

Gneiss obliquely long

Heiyun oblong gneiss is characterized by negative Eu anomalies, and even obvious positive anomalies (Eu = 0.60 1.45), strong Ba positive anomalies, and no obvious Sr anomalies, which are significantly different from other types of gneiss. . Its overall geochemical characteristics are similar to the acidic endmembers of Neoproterozoic bimodal volcanic rocks on the northern margin of the Yangtze Craton.

Gneiss II Long Granite

Similar to the Heiyun oblong gneiss, the fractionation of light and heavy rare-earth elements in Erchang granite gneiss is also strong (La_N / Yb_N = 6.3 ~ 17.2).

However, it is characterized by a strong negative Eu anomaly (Eu = 0.27 to 0.54), a strong negative Sr anomaly, and a weak negative Ba anomaly. Obviously different from the black gneiss gneiss.

A Gneiss Type A Granite

The rare earth pattern and spider web pattern of A-type granite gneiss are similar to those of Erchang granite gneiss, except that the former has relatively weak negative anomalies of Nb and Ta. The _ (Nd) (t) values of several types of gneiss in the area are not very different, reflecting that there may be a genetic relationship between their protoliths.

Gneiss similarities and differences

Their Nd isotope model ages T_ (DM) are concentrated around 2.0 Ga, indicating that their source area is mainly crustal material of Early Proterozoic, and the lithology is relatively uniform. It is speculated that the protoliths of Erchang granitic gneiss and Heiyun oblique gneiss are homogeneous and heterogeneous, and A-type granite is dehydrated by sulphite after the formation of large-scale type I granite The remaining crustal material was partially melted again. ^[3]