Allanite Petrochronology in Fresh and Retrogressed Garnet–Biotite Metapelites from the Longmen Shan (Eastern Tibet)
Résumé
Linking the timing of allanite growth to metamorphic conditions in metapelites is particularly chal-lenging because of the large variety of allanite textures and chemical compositions. This studyfocuses on five garnet–biotite metapelites retrogressed to different extents, from the internal do-main of the Longmen Shan (eastern Tibet) from which few petrochronological data documentingthe metamorphic ages are available. Microstructural observations were combined with whole-rockcompositions, detailed mineral characterization, phase equilibria modelling, andin situU–Th/Pb al-lanite dating to gain insights into the allanite reactivity relative to other rock-forming minerals. Allsamples experienced similar peak temperatures of 560–600C. Allanite is the main rare earth elem-ent (REE)-bearing accessory mineral but it exhibits different textures, such as epidote rims with dis-tinct REE contents as well as late inclusion-like dissolution features. Garnet is, along with allanite,critical to reconstruct the REE budget of these rocks. In the two samples where allanite is observedas inclusions in garnet, garnet shows no textural zoning and a low Y content (<90 ppm). This isattributed to early Y fractionation in allanite and epidote rims with no further equilibration with gar-net. This allanite is therefore pre-garnet (T<520C). In the sample where allanite is observed onlyin the matrix, both garnet and allanite–epidote rims exhibit more complex textural and compos-itional zoning, with a higher Y content in garnet. The Y incorporation in garnet is attributed to aseries of reactions involving allanite, interpreted as syn- to post-garnet growth (T>520C). Thisrelative chronology is confirmed byin situU–Th/Pb allanite dating: pre-garnet allanite shows agesofc.200 Ma, whereas syn- to post-garnet allanite has ages ofc.180 Ma. The timing of allanite ap-pearance strongly correlates with the biotite-in reaction predicted by the models and observed inmicrostructures. In the two samples collected close to the major Wenchuan Shear Zone—deformedup to mylonitization and retrogressed under greenschist-facies conditions (3–461 kbar, 350–400C)—allanite still preserves the peak metamorphic ages (c.200 andc.180 Ma). Allanite and itsepidote rims, however, are fragmented and partly replaced by a retrograde assemblage of QzþPb-depleted (<100 ppm) monazite, providing an age<90–100 Ma. Our results allow different meta-morphic stages for the studied transect in the central Longmen Shan to be refined: a prograde pathatc.200 Ma, a thermal relaxation and exhumation fromc.180 Ma, and a late greenschist overprint.This study also shows that when allanite is the principal phase in a mineral assemblage datable byU–Th/Pb, detailed observations of allanite textures may be highly informative in constraining thetiming of its growth relative to other major mineral phases.
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