Internal heating can cause melting, metamorphism, and crustal weakening in convergent orogens.This study evaluates the role of radiogenic heat production (RHP) in a Neoproterozoic ultrahigh-temperature metamorphic (UHTM) terrane exposed in southern Madagascar. Monazite and zircon geochronology indicates that the Paleoproterozoic Androyen and Anosyen domains (i) collided with the oceanic Vohibory Arc at~630 Ma, (ii) became incorporated into the Gondwanan collisional orogen by~580 Ma, and (iii) were exhumed during crustal thinning at 525-510 Ma. Ti-in-quartz and Zr-in-rutile thermometry reveals that UHTM occurred over >20,000 km 2 , mostly within the Anosyen domain. Assuming that U, Th, and K contents of samples from the field area are representative of the middle to lower crust during orogenesis, RHP was high enough-locally >5 μW/m 3 -to cause regional UHTM in <60 Myr. We conclude that, due in large part to the stability and insolubility of monazite at high crustal temperatures, RHP was the principal heat source responsible for UHTM, obviating the need to evoke external heat sources. Focused RHP probably thermally weakened portions of the middle crust, gravitationally destabilizing the orogen and facilitating thinning via lateral extrusion of hot crustal sections.

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Please cite this article as: Holder, Robert M., Hacker, Bradley R., Kylander-Clark, Andrew R.C., Cottle, John M., Monazite trace-element and isotopic signatures of (ultra)high-pressure metamorphism: Highlights• U-Pb dates and HREE indicate monazite recrystallization at UHP.• UHP monazites have high Sr and non-radiogenic Pb.• High Sr-Eu 2+ -Pb may result from feldspar breakdown at high pressure. AbstractMonazite U-Pb and trace-element data were gathered from six high-to ultrahigh-pressure (UHP) samples from the Western Gneiss Region, Norway, using LASS (laser-ablation split-stream ICP-MS) to investigate variations in monazite composition during high-pressure metamorphism. The UHP monazites were found to contain up to 7600 ppm Sr, 110 ppm non-radiogenic Pb, relatively weak negative Eu anomalies, and Y concentrations as low as 500 ppm. Amphibolite-facies monazite that rims the UHP monazite in one sample contains Y concentrations up to 1.6 wt %, Sr as low as 13 ppm, and no detectable non-radiogenic Pb. The UHP monazite composition (high Sr-Pb) is interpreted to result from growth in the absence of feldspar, possibly aided by increased compatibility of Sr-Pb-Eu 2+ in the monazite crystal structure at high pressure. Sr in monazite as a proxy for feldspar stability may be a useful tool not only in studying high-pressure metamorphism, but also in determining timescales of melting and crystallization, when the amount of feldspar changes over time.

The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatitemagmatite core rimmed by a lower-grade metamorphic envelope of andalusite and cordieritebearing schists. The oldest structure in the migmatite-magmatite core is a sub-horizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn-tectonic, with up to several tens of metres thick granitoid sills. Sillimanite-ilmenite-magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6-7 kbar at 710-780°C. Reorientation of sillimanite into the S2 fabric indicates that the S1-S2 fabric transition Accepted ArticleThis article is protected by copyright. All rights reserved. occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3-4 kbar and 680-750°C during development of the S2 steep fabric, and postectonic andalusite indicates further decompression to 2-3 kbar and 600-650°C. Widespread crystallization of post-tectonic muscovite is explained by the release of H 2 O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5-3 kbar and 500-530°C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5-3 kbar and 530-570°C; replacement of cordierite by fine-grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3-4 kbar and 540-620°C. Monazite from a migmatite analyzed by LASS yielded elevated HREE concentrations. The grain with the best-developed oscillatory zoning is 356±1. Pb), considered to date the crystallization from melt in the cordierite stability around 680°C and 3.5 kbar, whereas the patchy BSE-dark domains give a date of 347±4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest subhorizontal fabric is associated with the onset of magmatism and peak of P-T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite-migmatite core with supracrustal rocks. This study provides information about tectono-thermal history and time scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, ...