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 Hima layan 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 Hima layan sequence. Metamorphic monazite ages in the Greater Hima layan sequence also increase structurally upward: 16-21 Ma for sub solidus growth at the base of the Greater Himalayan sequence to ~25 Ma for peak-T metamorphism and anatexis near the top of the Greater Hima layan 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 Hima layan 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.

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.

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 intruand suggest a volcanogenic source, (5) traceelement chemistries of orthogneisses and meta basalts 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. GEOLOGIC SETTING AND STRATIGRAPHY

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