Magmatic, Metamorphic and Structural History of the Variscan Lizard Ophiolite and Metamorphic Sole, Cornwall, UK

This is the final version. Available on open access from Wiley via the DOI in this recordData availability statement: The field data, electron-microprobe analysis data, the CA-ID-TIMS and laser ablation U-Pb data and XRF data used for structural analysis, geochronology, geochemistry and thermobarometry in the study are currently being archived at Oxford Research Archive for Data (ORA-Data). It can be accessed here: https://doi.org/10.5287/bodleian:0zwDDM7QmThe Lizard ophiolite, Cornwall, South-West England, is the largest and best-preserved ophiolite within the Variscan orogenic belt. It forms part of the Rheic-Rhenohercynian suture zone, and was obducted northwestward onto the passive continental margin of Avalonia (Laurussia) during the Middle Devonian. It comprises an almost complete thrust slice of oceanic crust with sheeted dykes, gabbros, Moho transition sequence, and upper-mantle peridotites, underlain by a metamorphic sole. Despite the importance of the Lizard ophiolite in understanding Variscan tectonics, the origin and age of the Lizard ophiolite are debated. We present new field observations, structural maps and cross-sections of the Lizard ophiolite from extensive re-mapping, integrated with U–Pb geochronology, petrology, thermobarometry, and whole rock geochemistry. We report new U–Pb zircon (CA-ID-TIMS and LA-ICPMS) ages of 386.80 ± 0.25/0.31/0.52 Ma (Givetian) from a plagiogranite dyke intruding the Crousa Gabbros at Porthoustock, and 395.08 ± 0.14/0.22/0.47 Ma (Emsian) from partial melts of the metamorphic sole Landewednack Amphibolites at Mullion Cove. These ages, respectively, precisely date the formation of the Lizard ophiolite oceanic crust, and the age of cooling post peak-metamorphism of the sole. Petrological modeling on the Landewednack Amphibolites suggests peak metamorphic conditions of 10 ± 2 kbar and 600 ± 75°C. We demonstrate that the Lizard ophiolite formed as a supra-subduction zone ophiolite overlying an inverted metamorphic sole, and we combine our observations and data into a new geodynamic model for the formation and obduction of the ophiolite. The current data supports an induced subduction initiation model.Natural Environment Research Council (NERC)University of OxfordLeverhulme Trus

On the enigmatic mid-Proterozoic: Single-lid versus plate tectonics

The mid-Proterozoic (ca. 1850–850 Ma) is a peculiar period of Earth history in many respects: ophiolites and passive margins of this age are rare, whereas anorthosite and A-type granite suites are abundant; metamorphic rocks typically record high thermobaric (temperature/pressure) ratios, whereas ultrahigh pressure (UHP) rocks are rare; and the abundance of economic mineral deposits features rare porphyry Cu-Au and abundant Ni-Cu and Fe-oxide Cu-Ag (IOCG) deposit types. These collective observations have been used to propose that a stagnant-lid, or single-lid, tectonic regime operated at this time, between periods of plate tectonics in the Paleoproterozoic and Neoproterozoic. In our reappraisal of the mid-Proterozoic geological record, we not only assess the viability of the single-lid hypothesis for each line of evidence, but also that of the plate tectonic alternative. We find that evidence for the single-lid hypothesis is equivocal in all cases, whereas for plate tectonics the evidence is equivocal or supporting. We therefore find no reason to abandon a plate tectonic model for the mid-Proterozoic time period. Instead, we propose that the peculiarities of this enigmatic interval can be reconciled through the combination of two processes working in tandem: secular mantle cooling and the exceptionally long tenure and incomplete breakup of Earth's first supercontinent, where both of these phenomena had a dramatic effect on lithospheric behaviour and its resulting imprint in the geological record

Supporting Data for The Cycladic Blueschist Unit on Tinos, Greece: Cold NE Subduction and SW-Directed Extrusion of the Cycladic Continental Margin under the Tsiknias Ophiolite

Data collected during Tom Lamont DPhil

The tectonic and metallogenic framework of Myanmar: A Tethyan mineral system

Myanmar is perhaps one of the world's most prospective but least explored minerals jurisdictions, containing important known deposits of tin, tungsten, copper, gold, zinc, lead, nickel, silver, jade and gemstones. A scarcity of recent geological mapping available in published form, coupled with an unfavourable political climate, has resulted in the fact that, although characterized by several world-class deposits, the nation's mineral resource sector is underdeveloped. As well as representing a potential new search space for a range of commodities, many of Myanmar's known existing mineral deposits remain highly prospective. Myanmar lies at a crucial geologic juncture, immediately south of the Eastern Himalayan Syntaxis, however it remains geologically enigmatic. Its Mesozoic-Recent geological history is dominated by several orogenic events representing the closing of the Tethys Ocean. We present new zircon U-Pb age data related to several styles of mineralization within Myanmar. We outline a tectonic model for Myanmar from the Late Cretaceous onwards, and document nine major mineralization styles representing a range of commodities found within the country. We propose a metallogenetic model that places the genesis of many of these metallotects within the framework of the subduction and suturing of Neo-Tethys and the subsequent Himalayan Orogeny. Temporal overlap of favourable conditions for the formation of particular deposit types during orogenic progression permits the genesis of differing metallotects during the same orogenic event. We suggest the evolution of these favourable conditions and resulting genesis of much of Myanmar's mineral deposits, represents a single, evolving, mineral system: the subduction and suturing of Neo-Tethys

Monazite geochronology and petrology of kyanite- and sillimanite-grade migmatites from the northwestern flank of the eastern Himalayan syntaxis

A combined geochronological and petrological study of pelitic migmatites from the northwestern flank of the eastern Himalayan syntaxis has constrained the timing and P-T conditions of two high-grade metamorphic events that affected the south Lhasa block (Asian margin) and provides new insight into the tectonothermal evolution of the India-Asia collision. U(-Th)-Pb dating of in situ monazite shows that upper amphibolite-facies sillimanite-grade metamorphism and consequent partial melting occurred between c. 71 and 50 Ma at P-T conditions above 6.3 ± 1.2 kbar and 750 ± 30 °C. Further partial melting at upper amphibolite-facies kyanite-grade conditions occurred between c. 44 and 33 Ma at minimum P-T conditions of 10.4 ± 1.0 kbar and 698 ± 20 °C. These data are interpreted to record a south Lhasa block mid-crustal sillimanite-grade melting event in the Late Cretaceous to Early Eocene related to regional heat advection caused by coeval and prolonged emplacement of Gangdese batholith units. This was followed by a higher pressure and lower temperature kyanite-grade melting event during the Middle Eocene to Early Oligocene associated with deformation and crustal thickening in the south Lhasa block, coeval with kyanite-grade metamorphism along the Himalaya, as a result of the on-going India-Asia collision. These partially-melted crustal lithologies offer potential sources (or otherwise analogs for sources) for the Miocene emplacement of adakitic intrusions previously documented in the eastern Himalayan syntaxis region. © 2013 International Association for Gondwana Research

The Juvenile Hafnium Isotope Signal as a Record of Supercontinent Cycles

Hf isotope ratios measured in igneous zircon are controlled by magmatic source, which may be linked to tectonic setting. Over the 200-500 Myr periodicity of the supercontinent cycle - the principal geological phenomenon controlling prevailing global tectonic style - juvenile Hf signals, i.e. most radiogenic, are typically measured in zircon from granites formed in arc settings (crustal growth), and evolved zircon Hf signals in granites formed in continent-collision settings (crustal reworking). Interrogations of Hf datasets for excursions related to Earth events commonly use the median value, however this may be equivocal due to magma mixing. The most juvenile part of the Hf signal is less influenced by crustal in-mixing, and arguably a more sensitive archive of Earth's geodynamic state. We analyze the global Hf dataset for this juvenile signal, statistically correlating supercontinent amalgamation intervals with evolved Hf episodes, and breakup leading to re-assembly with juvenile Hf episodes. The juvenile Hf signal is more sensitive to Pangaea and Rodinia assembly, its amplitude increasing with successive cycles to a maximum with Gondwana assembly which may reflect enhanced subduction-erosion. We demonstrate that the juvenile Hf signal carries important information on prevailing global magmatic style, and thus tectonic processes

Age and anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault

The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32-12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation-inductively coupled plasma-mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: A complex Triassic-Jurassic (ca. 215-159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14-5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ~62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian-Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau