Comparision of P-T paths for Archean and Paleoproterozoic terrene and tectonic implications

Abstract

Metamorphic rocks provides us substantial information about the Precambrian geology setting from its mineral assemblages, mineral texture and the age of minerals. Metamorphic stages experienced by a particular rock in different time intervals with wide variety of pressures and temperatures could be generated and linked together to stipulate the Pressure-Temperature (“P-T”) path. Through studying the P-T path of those rocks, it indicates whether the rock is undergone burial or heating in the prograde period, and decompression or cooling during the retrograde. This research focuses on the study of the P-T path including its direction, whether it is clockwise or anticlockwise, of metamorphic rocks formed in both Archean and mainly Paleoproterozoic by explaining the kinds of tectonic process leading the rock formation. Total 63 numbers of P-T paths located in all major plates are being studied as to provide an accurate deduction, including the location of present Australia, Baltic region, Asia, North America, Africa, South America, Siberia, Greenland and India. By the accomplishment on study the numerous P-T paths including its pressure, temperature and the direction of the metamorphic rocks in major plates during early earth history, we know that: (i) Majority of metamorphic rocks are formed starting in late Archean and Paleoproterozoic with the earliest record at 2.8 -2.7 Ga, which indicates substantial tectonic events were undergone in those periods, i.e. the onset of plate tectonics. (ii) Clockwise P-T path dominates in the Paleoproterozic period, which can be explained by a series of collision of between the continental plates, or volcanism of the island arc or subduction dealing with oceanic plate and continental plate (iii) Vast amount of continental crust are generated between the Neoarchean and the Paleoproterozoic by collision shown by its clockwise direction of P-T path, which coincides with the accretion of supercontinent (iv) Anticlockwise P-T path exists more at the Neoarchaean than the clockwise one, which shows the occurrence of magmatic event happened at that time. The possible causes of the magmatism are due to early stage of plate tectonics, such as thinning of plates by rifting, arc related magmatism in stage of subduction.

(v) Generally, clockwise P-T path refers to burial event during collision or subduction, while the anticlockwise P-T path refer to thermal related event like contact metamorphism. Exception cases do exist as the region may coeval suffers from both regional and contact metamorphism, the direction of P-T explains whether pressure or temperature affects the metamorphism the most. (vi) For the analyze of P-T path, it is better not only focus on the direction of P-T paths, but also the peak temperature and pressure, thermal gradient. Combination with minerals assemblages and textural studies with P-T path analysis will give us more accurate tectonic environment for the formation of metamorphic rock.