Campaign-style U-Pb titanite petrochronology:Along-strike variations in timing of metamorphism in the Himalayan metamorphic core

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core(HMC), in order to document and investigate the causes of along-strike variations in its Oligocene-Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya.This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ～30 Ma to 12 Ma, recording(re-)crystallization between 625 ℃ and 815 ℃. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen.Disparities in the duration and timing of metamorphism within the HMC are best explained by alongstrike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes.These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger,thicker crystalline core in the eastern Himalaya.

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland,USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound,Ontario(NW)to Ft.Ann,New York(SE),including the younger,adjacent Ordovician Taconic allochthon.Fifty four carbonates(marbles,calcite veins,Ordovician limestone)were collected resulting in 68 strain analyses on mechanically twinned calcite(n=2337 grains)across the Central Gneiss Belt(CGB;3 samples),the Central Metasedimentary Belt(CMB;27 samples),the Central Granulite Terrane(CGT;Adirondack's;13 samples)and the Ottawan Orogenic Lid(OOL;11 samples).Twinning strains in the greenschist-grade OOL marbles preserve N-S shortening and U-Pb titanite ages(~1150 Ma;n=4)document these marbles formed during the Shawinigan(1190-1140 Ma)part of the Grenville orogen.From northwest to southeast,the Ottawan(1095-1020 Ma)twinning strain is dominantly a layerparallel shortening fabric oriented N-S(Parry Sound),then becomes parallel to the Grenville thrust direction(NW-SE)across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel(SW-NE).Within the regional sample suite there are two areas studied in detail,the Bancroft shear zone(n=11)and a roadcut on the southeast side of the Adirondack Mountains(Ft.Ann,NY;n=8).Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae(e_1 and e_2).The better-developed e_1 sets(n=406)record a horizontal fabric oriented NW-SE whereas the younger e_2 lamellae(n=146)preserve a margin-parallel(SW-NE)horizontal fabric.Both the e_1 and e_2 strains record an overprint vertical shortening strain(NEV),perhaps related to extensional orogenic collapse.We also report an Ottawan orogen-aged granoblastic mylonite(1093 Ma,U-Pb zircon;1102 Ma Ar-Ar biotite)in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.