Evolutionary Aspects of Archean Kolli-Massif, Southern India : An Archive of Crustal Processes

The continental crust is the record of the history of the Earth, of the processes and events that have contributed to the planet's evolution. It is now understood that the continental crust is growing continuously since the early ages of the Earth. Archean-Proterozoic boundary marks one of the major transition periods in the crustal evolution processes. However, there are only few crustal remnants available to investigate this milestone of Earth history, reported with significant chemical discontinuity. The Neoarchean crustal fragments of southern India provide a window to probe the processes that happened during such transitions. The geology of southern India can be broadly divided in to the Archean Dharwar Craton (DC) of granites and greenstones belts to the north and an assembly of crustal blocks experienced granulite grade metamorphism to the south from Archean to Neoproterozoic, namely the Southern Granulite Terrain (SGT). The relationship between DC and SGT terranes are not well established, primarily due to lack of studies on the growth and evolution on each of the crustal blocks. This study focuses on the crustal tract between Salem Attur Shear Zone and the Cauvery Shear Zone of the SGT. This region lies to the east of Palghat Cauvery Shear System, which is considered as dextral shear zone, suture zone, Neoproterozoic terrain boundary and reworked Archean crust in the previous studies. However, so far no comprehensive studies had been reported from the region that consists of a spectrum of rocks charnockite, granitic gneiss, hornblende gneiss, granite and mafic-ultramafics litho-units inclusive of a layered complex. The objectives of this study are 1) to understand the crustal formation processes in Kolli-massif 2) to delineate the chronology of events or processes through radiometric dating. 3) to understand the crustal reworking and evolutionary processes in Kolli-massif . Major tools used in this study include petrology (field studies and petrography), geochemistry, U-Pb Zircon geochronology, Sr-Nd and Hf Isotopes. The content of this thesis is divided in to six chapters. Chapter 1 is an introduction to the topic – crustal growth. It discusses the importance of continental crustal process in understanding the evolutionary history of the 2500 Ma Earth. It also emphasizes on the reason to investigate Kolli-massif which is a part of the Southern Granulite Terrain. Chapter 2 deals with the literature review which is relevant in the context of the study. The chapter discusses topics like structure of the Earth crust, various models proposed on the generation of continental crust (continuous as well as episodic) and also the models discussed in the literature on the generation of TTG (subduction of oceanic crust and ocean plateau and non-subduction). An overall view on crustal reworking and recycling is also included. The chapter ends with a short review on southern Indian crustal tectonics and a detailed discussion on the evolution Palghat Cauvery Shear Zone. Chapter 3 describes the geology of the study area Kolli-massif in details. This includes the structural, lithological units, field relation and geochronolgical aspects combined and their implications on the crustal assembly of southern India. Chapter 4 is a discussion on the results, interpretation and implications of crustal generation and evolution of the Archean Kolli-massif. This chapter is subdivided to four. Chapter 4.1 deals with possible source and tectonic settings for the magma generation which lead to the formation of Archean Sittampundi Complex. The whole rock and spinel chemistry two different suggests both MORB and arc signature for these rocks. Although this is such a quite contrasting scenario, such scenarios are known to occur in an intra-oceanic subduction in the Archean as well as modern analogue. The search for MOR setting lead to Kanjamalai, where major rocks like metagabbro show geochemical affinity, as described in Chapter 4.2. The presence of rocks like plagiogranite also supports MORB affinity. Based on field observations and above evidences Kanjamalai complex is interpreted as subducted remnant of an Archean Mid Oceanic Ridge. Chapter 4.3 deals with the major rock type of the region charnockite and granitic gneiss. The whole geochemical chemistry suggests arc signatures (depleted HFS elements, enriched LREE) and negative Nd and Hf isotope suggests reworked magma. However, the high HREE content and absence of Eu anomaly in the charnockite but reverse case of granitic gneiss indicates they might have of a different source and may not solely by the subduction of oceanic crust described in chapter 4.1. Combining the results from Hf and Nd isotopes that shows the presence of an older crust of age 2700-2900 Ma, it can be concluded that the an older oceanic crust, probably with an ocean plateau was part of subduction and magma genesis. The presence of garnet websterite describes accretion in operation in the generation of Kolli-massif. Chapter 4.4 deals with crustal recycling. The results on the investigation on meta-BIFs yielded results that can be interpreted that the iron formations were deeply subducted. The proposal of accretionary tectonics is also supported by the presence of meta-BIFs in the shear zone with in the Kolli-massif. Chapter 5 deals with the Neoproterozic reworking of the Archean Kolli-massif. The investigations on the sapphirine bearing granulite suggest that the rocks have undergone UHT metamorphism (6Kbar and 925˚C). The geochronogical evidences shows that the zircon rim growth ca. 550 Ma over a 2480 Ma crust. This suggests crustal reworking that would have happened during the Gondwana amalgamation happened during the Neoproterozoic time.It is therefore concluded in Chapter 6 that the Kolli-massif is having an Archean nucleus that was grown by the arc accretion. This reworked during the regional metamorphism along with the Gondwana metamorphism in the Neoproterozoic. Further scope of this study is also discussed