4.5 Geochemistry

Open Research Online The Open University’s repository of research publications and other research outputs The Thermal, Metamorphic and Magmatic Evolution of a Rapidly Exhuming Terrane: the Nanga Parbat Massif, Northern Pakistan. Thesis How to cite: Whittington, Alan Geoffrey (1997). The Thermal, Metamorphic and Magmatic Evolution of a Rapidly Exhuming Terrane: the Nanga Parbat Massif, Northern Pakistan. PhD thesis The Open University. For guidance on citations see FAQs. c 1997 Alan Geoffrey Whittington https://creativecommons.org/licenses/by-nc-nd/4.0/ Version: Version of Record Link(s) to article on publisher’s website: http://dx.doi.org/doi:10.21954/ou.ro.0000fe71 Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk The thermal/ metamorphic and magmatic evolution of a rapidly exhuming terrane: the Nanga Parbat Massif, northern Pakistan. A thesis accepted for the degree pf Doctor of Philosophy by Alan Geoffrey Whittington, M.A. (Cantab.) Department of Earth Sciences, The Open University August 1997 /: ProQuest Number: C652148 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest C652148 Published by ProQuest LLO (2019). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLO. ProQuest LLO. 789 East Eisenhower Parkway P.Q. Box 1346 Ann Arbor, Ml 48106- 1346 A bstr a c t The Nanga Parbat-Haramosh Massif (NFHM) is the most northerly outcrop of Indian Plate rocks in the Himalayan orogen, exposed by rapid recent exhumation through the structurally overlying Kohistan Island Arc in northern Pakistan. Exhumation has been achieved by erosion in the apparent absence of extensional tectonics. Metabasaltic dykes intruded into polymetamorphic basement rocks provide time markers that distinguish early from late fabrics that probably correlated with Precambrian and Himalayan deformation respectively. Nanga Parbat is anomalous within the Himalayan orogen for both the timing and nature of metamorphism and anatexis. A multidisciplinary approach allows the thermal history to be tightly constrained. Metamoiphism is characterised by cordierite growth during near-isothermal decompression. Garnet inclusion assemblages record P-T conditions of 720 ± 50’C and 7.5 ±1.5 kbar, while spinel-cordierite intergrowths unique to the massif interior formed at 710 ± 60*C and 5.0 ± 1.1 kbar. Lower grade assemblages are exposed towards the massif margins. Tourmaline leucogranite plutons and sheets are characterised by similar trace element geochemistry to Miocene High Himalayan granites, indicative of vapour-absent crustal melting as would result from rapid decompression, but were emplaced more than 10 million years later. More recently, fluid infiltration into conjugate shear zones accommodating vertical stretching in the core of the massif resulted in anatexis and the formation of restitic cordierite-bearing pods. ô^r-^Âr studies indicate regional cooling below 400’C as recently as 1 Ma, and the distribution of excess Â r in basement samples is indicative of magmatic/metamorphic fluids at depths > 10 km. One-dimensional thermal modelling indicates that increasing cooling rates through time do not require increasing exhumation rates because advection of heat results in a steepened near-surface geotherm. These models predict both the style and timing of metamorphism and anatexis in this rapidly exhuming terrane, and are consistent with petrological and geochemical constraints determined from the NPHM basement. FOREWORD Bonsai Mountains Just any old piece of rock won't do. The trick is to spot the mountains while they are young. In a sense the expert is selecting that piece of rock which, after the normal processes of continental collision and upheaval, will be at the very peak - i.e., from one pont of view, the rock with the correct morphic field which will cause the rest of the mountain to form underneath it.... (from "The Discworld Companion", by Terry Pratchett and Stephen Briggs, 1994, Cassell, London) 3 ^ '"Zc. w UIM AMt^ ÇTKMVC W üp NAfT^^ Pa RS^JT". Jl4A *^M/Î5 Acknowledgements I am very grateful to many people who have contributed directly and indirectly to the completion of this thesis. My supervisors, Nigel Harris and Rob Butler, provided support and two amusingly different approaches to fieldwork. I must thank Nigel in particular for his fearless perusal of various manuscripts and chapters over the last 3 (and a bit) years. Drs. Mubarik Ali and Azam Khwaja in Islamabad are thanked for their hospitality, and assistance in exporting rocks. Jim Davis was a superb field assistant, showing restraint in the face of militant incompetence, and stoicism in the face of jiardia. Judy Baker, Dave Mattey, Martin Hand, Karen and Phil also provided interesting diversions during fieldwork. Ghulam Nabi, and extended family, provided diversions from fieldwork, and are thanked for providing traditional remedies for our various ailments. Abdullah Baig, expert in most aspects (cultural, scientific, culinary and pecuniary) of the Northern Areas, was an excellent host at the Hunza Inn, and never complained at our unusual night time antics. The involvement of various drivers, porters, guides, cooks and hangers-on is gratefully acknowledged (where appropriate!). I owe thanks also to numerous people who assisted me with various machines and labs, including Eric Condliffe (Leeds), Stephen Reed and Mike N. (Cambridge), Andy Tindle and Phil Potts, Nick Rogers, John Watson and Pete Webb, Holly Holbrook, Mike, Brian and Kay, while PVC and Mabs (just) tolerated my obscenely radiogenic samples in the isotope lab, and Simon Kelley and Jo Wartho made the argon work possible. Other work-related help came from Jessica Bartlett, Anthony Cohen, Janet Dryden, Mark George, Dave Peate, Rita Quill, Steve Reddy, Simon Turner, Derek Vance, and numerous Himalayan types at conferences. My best wishes to the next generation, Christophe, Sarah and Gavin. Well done to various housemates and office mates who have survived the Elric experience (and some who still talk to me) including Mike, Rhino, Orange, John, Johnny, Zelkjo, Arlene, Pete, Rob, and Yiming. Also to Linda, Rick, Ed, Alex, Fiona, Jugs, Ben and Fran, Mike, Bill, Treez, Paul and Jane, Sephs, Ant, and others who I never officially lived with but have fallen over my prostrate form on their floor at various times. Farewell to Beatrice, who didn't survive the Elric experience and cost me my no claims bonus. Love to all the departmental babes, especially Butty "not a thug", Claire "not a bimbo", Linda "Hello young man", Liz and Johrmy, Lou, Phil "Blando", Rob "Kula Shaker", Russ "5 years" and Yvette. Thanks also to members of the OU Club and PGSS committees, all the bar staff, and anyone I've missed. "Aloha " to H, co-worker in the "Warp Toilet" aural adventure, and respect to Mike Bullivant for asking me to play my records at other people, to Madcap Gordon for wearing mustard cords, Skaz/Slunk and Camilla, Simon, Manna Maya, and the more famous ones... The Mad Kows have been a source of misplaced pride for a couple for years now, SO large moos and big ups to the Mad Kow posse: Ed (the enforcer), Carolyn, "end zone" Andy, Sarah and Thomas, Babs, John B, John D, Trevor, Tim, Mandy, Loz, Speds, Teri, Sara, Leroi, Johnny, American Martin, Martian Martin, Fred, Aaron, Linda, Cads, various Australian internationals and anyone else who has ever chased the plastic disc. Thanks also to my parents and Richard who provided encouragement during writing, and constant reminders of the clean-cut and besuited fate awaiting me should I falter in the last stages. At last I can dedicate an obscure academic work to them, for a change. Finally, huge hugs to Angela (aka DJ Boss) for providing much needed relief from work, liking the Sisters, putting up with plastic discs and other foibles, not (quite) biting my finger off, and moving to London just SO I wouldn't get distracted from writing.... T able o f C o n t e n t s Chapter 1 - Introduction 1 1.1 Aims of the project 1 12 Thesis structure 3 1.3 Geology of the Himalaya 5 1.3.1 Introduction 5 1.3.2 Timing and nature of collision 7 1.3.3 Post-coUisional evolution of the Himalaya 11 1.4 The western Himalaya 16 1.4.1 Kohistan 18 1.4.2 The Karakorum Block 25 1.4.3 The Indian Plate in Pakistan 27 1.4.4 Growth of the NPHM 31 1.4.5 Recent events 34 Chapter 2 - Field Relations And Petrography 37 2.1 Introduction 37 2.2 Previous work and structural geology 38 2.3 Lithologies of the southern NPHM 50 2.3.1 Kohistan 53 2.3.2 Massif Margins 54 2.3.3 Basement gneisses 59 2.3.4 Amphibolite dykes 69 2.3.5 Leucogranites 73 2.3.6 Cordierite-bearing leucosomes 79 2.4 Summary and metamorphism-deformation history 84 Chapter 3 - Metamorphism of the southern Nanga Parbat Massif 88 3.1 Introduction 88 3.1.1 Previous work 88 3.1.2 Thermobarometric techniques 96 3.2 Thermobarometry 98 3.2.1 Pelitic lithologies 99 3.2.2 Calc-siHcate lithologies 106 3.2.3 Amphibolite sheets 115 3.3 Garnet zoning patterns in metapelites

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