Zircon as a Recorder of Trace Element Changes during High-Grade Metamorphism of Neoarchean Lower Crust, Shevaroy Block, Eastern Dharwar Craton, India

Abstract

Systematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry, are seen along a ca. 95 km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidized granulite-facies rocks near the magnetite-hematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-facies charnockite. Cathodoluminesence (CL) and back scattered electron (BSE) sub-grain imaging and Sensitive High-Resolution Ion MicroProbe (SHRIMP) analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasing depleted compositions between 2530 and 2500 Ma . With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted magmatic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs. granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally-derived, oxidizing, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.

Introductory Chapter: Introduction to New Advances in Metamorphic Geology

“…The main advances in this field are centered around the application of new chemical systematics to follow mineral phase changes and link them to a geological process. High-resolution microchemistry is commonly performed to lower the potential resolution of time scales of the metamorphic events recognizable; hence, a strong drive is visible for developing new techniques to determine the ages of various metamorphic events in the micromineralogy level [11][12][13][14]. As metamorphism may occur over a wide range of temperature and pressure conditions, the applicable chemical techniques to define these conditions have formed a great variety of research avenues over the past decades.…”

Local Structure, Structural, Vibrational, and Optical Properties of Natural Zircon

“…Such types of zircon crystals are found in several countries of the world and also in India. [30][31][32][33] Researchers have mainly investigated the geological, compositional, etc. properties of zircon crystals, [34][35][36][37][38] although reports are available on the structural characterization using inductively coupled plasma-mass spectrometry (ICP-MS), total reflection X-Ray fluorescence (TXRF), X-Ray diffraction (XRD), Raman, Fourier transform infrared (FTIR), and UV-visible spectroscopies.…”