Stacey L. Corrie

The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate

The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, however, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales and suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and metabasalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses (“Ulleri”) likely represent shallow intrusions, not Indian basement.

The fall and rise of metamorphic zircon

Abstract Zircon geochronology and geochemistry are increasingly important for understanding metamorphic processes, particularly at extreme conditions, but drivers of zircon dissolution and regrowth are poorly understood. Here, we model Zr mass balance to identify P-T regions where zircon should dissolve or grow. Zirconium contents of major metamorphic minerals were assessed from published data and new measurements, and models were constructed of mineralogical development and zircon abundance for hydrous MORB and metapelitic compositions along representative P-T paths. Excluding zircon, the minerals rutile, garnet, and hornblende strongly influence Zr mass balance in metabasites, accounting for as much as 40% of the whole-rock Zr budget. Clinopyroxene and garnet contain more Zr than plagioclase, so breakdown of plagioclase at the amphibolite to eclogite facies transition, should cause zircon to dissolve slightly, rather than grow. Growth of UHP zircon is predicted over a restricted region, and most zircon grows subsequently at much lower pressure. In metapelites, zircon is predicted to undergo only minor changes to modal abundance in solid state assemblages. Partial melting, however, drives massive zircon dissolution, whereas melt crystallization regrows zircon. From a mass-balance perspective, zircon growth cannot be attributed a priori to the prograde amphibolite-eclogite transition, to UHP metamorphism, or to partial melting. Instead, zircon should grow mainly during late-stage exhumation and cooling, particularly during oxide transitions from rutile to ilmenite and melt crystallization. As predicted, most zircons from HP/UHP eclogites of the Western Gneiss Region and Papua New Guinea substantially postdate eclogite formation and maximum pressures.

Metamorphic history of the central Himalaya, Annapurna region, Nepal, and implications for tectonic models

Pressure-temperature-time (P-T-t) conditions of metamorphism have been determined in the Annapurna region of central Nepal that place new constraints on the structural and tectonic evolution of the Himalayan orogenic wedge. Peak P-T conditions increase structurally upward: ∼525 °C and 8 kbar in the Lesser Himalayan sequence, 650 °C and 12 kbar at the base of the Greater Himalayan sequence across the Main Central thrust, 750 °C and 12 kbar in the middle of the Greater Himalayan sequence, and 775 °C and 13 kbar near the top of the Greater Himalayan sequence. Metamorphic monazite ages in the Greater Himalayan sequence also increase structurally upward: 16–21 Ma for subsolidus growth at the base of the Greater Himalayan sequence to ∼25 Ma for peak-T metamorphism and anatexis near the top of the Greater Himalayan sequence. These ages are several million years older than at equivalent structural levels at Langtang, ∼200 km to the east. The P-T-t data recommend reinterpretation of the Bhanuwa fault within the Greater Himalayan sequence as a thrust, and the presence of a different thrust structurally above the Bhanuwa thrust, here named the Sinuwa thrust. The new data are consistent with progressive stacking of tectonic slices, with calculated overthrust rates that are consistent with some (but not all) models that presume ∼2 cm/yr convergence across the Himalaya since 25 Ma. Despite differences in absolute ages, similarities among the chemical systematics of monazite, peak P-T conditions, and overthrust rates calculated for Annapurna when compared to Langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers, although the timing and duration of movement on discrete thrust surfaces may differ.

The Malari Leucogranite, Garhwal Himalaya, Northern India: Chemistry, Age, and Tectonic Implications

In the Garhwal region, India, the Malari leucogranite cuts the South Tibetan detachment system, a large-scale normal fault system at the top of the High Himalaya. The leucogranite crosscuts ductile normal-sense shear fabrics and has experienced relatively little subsolidus brittle deformation or alteration. Its relatively evolved bulk chemical composition, high Rb/Sr ratio, and normative corundum indicate a (meta)sedimentary source, likely the underlying Greater Himalayan sequence. Zircon U-Pb ages, collected by laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) and corrected for initial U/Th disequilibrium, indicate emplacement at 19.0 ± 0.5 Ma. Thus, ductile normal shear on the South Tibetan detachment system must have ceased by 19 Ma. Studies elsewhere in the Himalaya suggest initiation of South Tibetan detachment system ductile movement not earlier than 24 Ma, and likely ca. 22 Ma. The short duration of extension (≤5 and likely ∼3 m.y.) and early cessation contrast with channel-flow models that predict long-duration ductile normal shear, and large displacements after ca. 20 Ma. Observations are instead better explained by critical taper models, in which internal weakening of the wedge, likely from partial melting, caused a brief interval of flattening and ductile extension in the rear of the wedge.

Resolving the timing of orogenesis in the Western Blue Ridge, southern Appalachians, via in situ ID-TIMS monazite geochronology

Both Taconian (ca. 450 Ma) and Acadian (ca. 380 Ma) ages of high-grade deformation and metamorphism have been advocated for the Western Blue Ridge, southern Appalachians, United States. To resolve this debate, metamorphic monazite from the Great Smoky Mountains was dated via in situ microsampling plus isotope dilution–thermal ionization mass spectrometry (ID-TIMS). U-Pb ages are 450 ± 5 Ma (±2σ error), and mean 207 Pb/ 206 Pb ages are 453 ± 10 Ma. These data indicate that peak metamorphism and high-grade deformation occurred during the Taconian orogeny, although they do allow for a low-grade (sub-amphibolite-facies) Acadian overprint. Unresolved issues remain regarding the comparability of U-Pb TIMS ages with those obtained via other methods.