MIGMATITES OF HIGHER HIMALAYAN CRYSTALLINES: THEIR ROLE IN COLLISION TECTONICS

Abstract

Himalaya is the youngest and evolving active mountain belt in the world displaying various geodynamic processes with the development of distinct tectonic units. This mountain belt extending laterally about 2500 km from west to east with 250-300 km width and providing an ample opportunity to study the exhumed rocks which have undergone deep burial, reactivation, remobilization, deformation, metamorphism and intrusion by the Tertiary Leucogranite due to the active collisional tectonics between Indian and Eurasian plate The Higher Himalayan Zone known as Higher Himalayan Crystallines (HHC), characterizes the northernmost exposed part of the Indian plate thrust southward over the Lesser Himalayan rocks along a major thrust called the Main Central Thrust (MCT) (also called Munsiari Thrust in Garhwal) with an estimated overthrusting of about 80-100 km. This MCT is about 2-5 km wide ductile shear zone and providing maximum crustal shortening of 250 km during collision, with reactivation afterward in different episodes. The northern boundary of the HHC is marked by the Trans Himadri Shear Zone (THSZ)/ Zanskar Shear Zone (ZSZ)/ Rohtang Shear Zone (RSZ)/ Martoli Fault (MF)/ Tethyan Thrust (TT)/ South Tibetan Detachment System (STDS) and its associated splays. The relationship among metamorphism, migmatisation, evolution of HHC and generation of leucogranite in the HHC with reference to the tectonic framework has been one of the major fields of interests in Himalayan geodynamics. Since the closure of the Indian and Eurasian plate about 57 Ma ago multiple phases of deformation, metamorphism, metamorphic fluid activation, compression, decompression, crustal loading and reloading has been n taking place. The occurrence of migmatite and leucogranite in this prevailing tectonic regime has left a good opportunity to evaluate the relationship between them and the Higher Himalayan Tertiary Leucogranite. The HHC of Alaknanda and Dhauliganga valley is defined by the MCT in the South and Malari Fault in the north divides the rocks of the HHC into two major units as upper Vaikrita Group comprising Joshimath, Pandukeshwar and Badrinath Formations and basal Munsiari/Tapoban Formation. The rocks of the Vaikrita Group comprises greenschist fades to higher amphibolite facies rocks, which include mica-, garnet-, staurolite-, kyanite-, sillimanite-schists and gneisses, sillimanite-Kfs grade rock, migmatite and leucogranite with intermittent thin bands of calc-silicate, amphibolite and veins of pegmatite and quartz while the Munsiari Formation comprises predominantly of quartzite and metabasite with intercalations of phyllite and chlorite schist. The pressure and temperature of the rocks from the two valleys have been attempted using THERMOCALC V-3.25 on the matrix (rim) and inclusion (core). Pressure temperature data and also garnet profiles clearly indicate a progressive PT path experienced by the lower part of the Vaikrita Group where rim temperature varies from 520±90°C to 708±40°C with core and rim values consistently indicating increase in temperature from ~600 to ~700°C and along with rim pressure increasing from ~7 to -12 kbar. Towards north, pressure gradually reduces while temperature initially shows slight reduction in the central part of the thrust sheet followed by slight increase in the migmatite zone, where the temperature reaches to about 700°C. The pressure on the other hand shows a total reduction of about 5-6 kbar from the Vaikrita Thrust to migmatite zone. The inclusion and matrix thermobarometry of the in migmatite zone shows an increase in temperature of about 30-50°C from core to rim while pressure reduces to about 2 kbar from core to rim. Trace elements and REE from 45 samples including metasediment, migmatite and leucosome and leucogranite clearly shows a similar pattern indicating the petrogenetic link among the rock types. The spider diagrams of the metasediment and the migmatite show similar pattern with negative anomalies of Ba, Nb, Sr, Zr and Ti; in addition to this Th and Y shows a slight positive anomaly. In the metasediments, Th is quite high as compared to U but gradually their pattern got reverse in migmatite and leucosome. The positive anomaly of Th and Y in the metasediment indicates the presence of accessory phases like monazite and apatite most of which retain in the restite during partial melting, therefore, shielding the elements from the melt and hence producing negative anomaly in the leucosome and leucogranite. The REE diagram of metasediment and migmatite shows a less fractionation in both LREE and HREE represented by a smooth pattern with slight positive Gd and Tm anomaly along with negative anomaly in Eu. The leucosome part on the other hand shows enriched pattern of HREE producing a slightly upward curve which can be attributed to the residual garnet in the leucosome. However, the concentrations of the REEs in the leucosome are largely reduced. The strong positive anomaly of Eu in leucosome indicates the accumulation of plagioclase in the leucosome which is one of the dominant mineral in leucosome samples. The heterogeneity in the REE in leucosome specially Sm, Nd and Eu are the clear indication of the disequilibrium melting or segregation. REE pattern of the leucosome can be attributed to metamorphic segregation, which differentiates minerals under subsolidus condition and can produce similar pattern with different concentration IV level of the elements. The similar REE pattern and the field evidences of large numbers of small pods along main foliation clearly indicate partial melting under H20 saturated condition as a suitable mechanism for the formation of migmatite and the generation of the leucogranite of Alaknanda and Dhauliganga valley. Isotopic studies indicate muscovite ages of Vaikrita Group, for both the valleys range between 20 to 14 Ma with biotite ages varying between 13 and 7 Ma. However, majority of the muscovite give ages between 20 to 17 Ma and biotite giving between 13 to 10 Ma range. The lower ages are yielded by the pelitic sample sandwiched between the quartzite bands of Pandukeshwar Formation, the migmatite and adjacent to the Vaikrita Thrust (VT). However, the muscovite ages of the samples adjacent to the VT do not vary from those of the migmatite zone, while the biotite ages change largely, justifying the remobilization of the LILE due the ductile deformation at MCT zone. MCT zone samples yield muscovite age of 8.61 Ma while biotite ages are as low as 1.43 ± 0.01 Ma. Similarly, the Munsiari Formation yields biotite ages between 2.07-2.05 Ma. The age pattern, across the HHC, show a sharp break in the mica cooling ages across the VT, i.e., between the Vaikrita and the Munsiari Formation. eNd and 87Sr/86Sr values have also been calculated at present day values, at 46 Ma value considering SHRIMP U-Pb age from Bhagirathi valley for re-homogenization age and at 500 Ma which has been considered by other workers as the time of re-homogenization. It is found that the calculated values at t=o and t=46 Ma, are more comparable with the range of the published data while the values calculated at t=500 Ma most of the samples resulted into positive values and their 87Sr/86Sr ratios are giving lesser values than the BABI (Basaltic Achondrite Best Initial). Therefore, for Vaikrita Group of HHC probably might have re-homogenized post-dating 500 Ma, which has been widely used for the Himalayan rocks. It appears that the rocks of the HHC were re-homogenized comparatively in recent time. The values of £ncj(46) and 87Sr/86Sr calculated at t=46 Ma gives clear separation of migmatite from mestasediment while the values at t=0 do not discriminate migmatite from metasediments. The average values of present day £Nd and 87Sr/86Sr gives about -16 and 0.901836 respectively while for Tethyan it is about -17 and 1.002341 and for Munsiari Formation the values are about -24 and 0.905278. Leucosome samples yield anomalous values of £Nd(0) due to high Sm/Nd ratio and present day 87Sr/86Sr ratio gives very high values indicating a disequilibrium melting of the crustal rocks. From the field observation and thin section study it reveals that the rocks of the Vaikrita Group and the Munsiari Formation experiences inverted metamorphism. The rim temperature and pressure of the lower Vaikrita Group increases from 520±90°C to 708±40°C and 6.8±2.0 to 11.4±1.1 kbar, while in the migmatite zone occurring at the northern upper part of the HHC shows clockwise P-T path. The substantial reduction in pressure of about 5-6 kbar (from Vaikrita Thrust to migmatite zone) and almost constant temperature (from Vaikrita Thrust to migmatite zone, with a small dip of temperature at the central part of the thrust sheet) along with the occurrence of small pods of melt along the foliation of the migmatite indicates near isothermal decompression causing partial melting of the metasediment and subsequent generation of leucogranite. The trace elements and REE for metasediment, migmatite and leucosome and leucogranite shows a very similar pattern indicating that source rock of the leucogranite could be from migmatite due to partial melting under H20 saturated condition. The changing concentration in the VI REE and few trace elements in the leucosome indicate the disequilibrium melting or segregation which is also supported by the highly anomalous values of Sm/Nd and 87Sr/86Sr. Calculated cooling rate considering the average muscovite and biotite ages from the Vaikrita Group and the Munsiari Formation clearly indicate two distinct patterns. The samples of the Vaikrita Group give a gentle path indicating moderate exhumation rate between -17 to -11 Ma. Whereas, basal Munsiari Formation indicating a very fast cooling and exhumation rate. Mineral assemblages, geochemical data and exhumation and/or cooling age history in addition to field character do indicate that migmatite leucosome and small volume of insitu tourmaline bearing leucogranite indicate cummulation rock from water-saturated melting of pelitic metasedimentary rocks. The formation of migmatite happened at around 46 Ma corresponding to peak metamorphic event due to collision tectonics of the Himalayan orogeny. At this time Sr-Nd isotopic ratio also got re-homogenized. The presence of feeder dykes for main tourmaline-bearing leucogranite indicate that the source for the main body could be migmatite which is also being supported by similarity in REE pattern of the main body could insitu tourmaline bearing leucogranite. The dyking occurred due to buoyancy or injection mechanism within the upper part of the HHC. After leucogranite formation, terrain exhumed separately with different exhumation rate, lower package being exhumed faster with overlying package as much slower rate. Within the upper package the central portion has exhumed at lesser rate as compared to the two flanks bounded by the Vaikrita Thrust in the south and the Malari Fault in the north. Vll ACKNOWLEDGEMENTS First and foremost I convey my deep indebtness to my supersvisor Dr. Sandeep Singh, Associate Professor, Indian Institute of Technology, for his stipulated guidance, unwavering support and encouragement. This thesis could not have attained its present form, both in content and presentation, without his active interest, direction and guidance. His unmatched excellence in the subject, bountiful energy, and personal care has been the source of inspiration. He has devoted his invaluable time and took personal care in motivating me till the final printout of the thesis. His personal counselling and word of encouragement were cheering whenever I was in the period of plummation. His valuable advices and suggestions remains a source of inspiration. I am also grateful to Prof. R. M. Manickavasagam, Retd. Head, Institute of Instrumentation Centre, IIT Roorkee for his valuable suggestions at various stages during geochemical analysis at IIC, IIT Roorkee. Provoking discussions and his inspiring presence helped me to ride over the problems and complete the analysis successfully. His helping and kindness nature remains a source of motivation. My sincere thanks are due to Prof. A. K. Choudhuri, Head Institute Instrumentation Centre, IIT Roorkee for his valuable suggestions and guidance during my geochronological analysis at Instrumentation Centre, IIT Roorkee. Interesting discussions and instructive suggestions and for his scholarly advice helps immensely during the course of analysis.

I express my sincere thanks to Prof. A. K. Jain, Earth Sciences, IIT Roorkee for his valuable advice and suggestions during my research. His unambiguous instruction and fruitful discussions with him helped me a lot for this research. I am greatly indebted to Dr. T. K. Ghosh, Instrument Instrumentation Centre, IIT Roorkee, for his sincere and active discussions during the EPMA analysis. It could not be possible to carry out the analysis without his copious help. I am greatful to Prof. H. Sinvhal and his team for helping in taking final printout. Literature provided by Prof. Nigel Harris (UK), Prof. Jibamitra Ganguli (USA), Prof. Randall Parish (UK), Prof. Spear Frank (USA), Prof. Michael Brown (USA), Prof. Derek Vance (UK), Prof Allan Whittington (USA), Prof. DeCelles (USA) and Prof. An Yin (USA) is deeply acknowledged. The non-teaching staffs of the Department of Earth Sciences are also gratefully acknowledged for their unstinted help in many ways. I am thankful to my labmates Dr. Rajeev Kumar, Soumayjit and James for their loving support and coordination making a healthy working environment. I also appreciate my colleagues Dr. Balaji, Dr. Harrish, Dr. Vivesh, Dr. Ratnadeep, Narendra, Ajaey, Manish, Anshuman, Avnish, Mathur, Krishna, Srikishna, Deepak, Acharrya, Pati, Laxman and others for their loving support during my stay and lifting morals always high. I owe my sincere thanks to, Kamalji, Rameshji, Handaji, Jawelji of IIC, for their helping hand during various stages of analysis at IIC. I am also thankful to Surender (TIMS, Lab) for his cheerful support at the TIMS lab and Bhimji (Earth Sciences) for his sincere effort during thin section preparation.

I am grateful to all those who have helped me directly or indirectly in the successful completion of this thesis. Word fails to express to thank my parents and Tado. I would have never come so far without their rock solid support and unconditional love. I whole heartedly dedicate this work to them.