Simultaneous operation of opposing reaction mechanisms: The influence of matrix heterogeneity on post‐kinematic garnet crystallization in an inverted metamorphic sequence

Abstract

Metapelites from the inverted Barrovian sequence in the Sikkim Himalaya (northeast India) are shown to be largely continuous with respect to their bulk rock compositions, microstructures and pressure–temperature–time–deformation (P–T–t–D) histories. However, the upper garnet–lower staurolite zone demarcates a region of microstructurally anomalous post‐kinematic garnet populations contained within strongly segregated matrices. The different microstructures within samples from this region cannot be attributed to differences in their thermobarometric histories or bulk compositions, but are instead interpreted to represent an otherwise unexposed level of the Daling Group that is now exposed along a post‐metamorphic thrust splay. These heterogeneous samples contain several discrete garnet populations that progressively crystallized with increasing P–T. Garnet populations that experienced the most protracted growth now form complex polycrystals that exhibit crystallographically controlled and morphologically irregular interfaces adjacent to micaceous and quartzofeldspathic domains respectively. Electron backscatter diffraction indicates that these polycrystalline garnet structures contain numerous coalesced porphyroblasts that are structurally uncorrelated across their grain boundaries. However, a crystallographically preferred orientation at the polycrystal scale is interpreted to derive from epitaxial crystallization of early‐formed garnet porphyroblasts on precursor mica. Later‐nucleated porphyroblasts within polycrystals preferentially concentrated towards quartzofeldspathic domains, with the overall nucleation distribution likely controlled by a complex interplay between chemical heterogeneities, strain partitioning and epitaxial crystallization. The subsequent growth of these polycrystals was equally spatially heterogeneous; it was moderated by differences in the efficiency of grain boundary transfer between quartzofeldspathic and micaceous domains that precluded thin section‐scale chemical equilibration. In contrast to samples from Sikkim containing more typical porphyroblastic populations in continuous and disseminated matrices, heterogeneous availability of garnet‐forming components within this strongly layered matrix is shown to have resulted in grain‐scale variations in growth rates and the spatial juxtapositioning of interface‐controlled microstructures and locally equilibrated chemical compositions with those that were transport controlled.

Preferred orientations of garnet porphyroblasts reveal previously cryptic templating during nucleation

“…5 , illustrating that ( ) gt and ( ) gt are within 16° of ( ) gt and that ( ) gt and (0 1 2) gt are 7° and 18° from (0 1 1) gt , respectively. This generally agrees with previous results 16 – 18 , 21 , suggesting that garnet nucleates epitaxially on the crystal structure of muscovite and/or biotite in the more exotic microstructures presented in those studies. Our results and interpretations further show that epitaxial nucleation is not restricted to unusual microstructures, but instead may be important during crystallization of many or most regional and contact metamorphic rocks.…”

“…S3-7), so inheritance of garnet orientation from muscovite resulted in parallelism with chlorite. These interpretations agree well with previous studies that interpret that garnet can nucleate on the crystal structure of muscovite and/or biotite [16][17][18]21 , with these previous studies showing (1 1 0) gt or ( d-i) Crystal structure models of (0 7 9) gt, (100) ms , ( − 7 7 9) gt, ( − 1 1 6) gt , (1 0 0) ms , and (1 0 0) gt , with dashed lines highlighting relationships between Al in each. The inset shows the 3D orientation of all of the crystal planes.…”

“…These interpretations agree well with previous studies that interpret that garnet can nucleate on the crystal structure of muscovite and/or biotite 16 – 18 , 21 , with these previous studies showing (1 1 0) gt or ( ) gt being within 20° of parallel to (0 0 1) ms,bt . Our results show a clustering in which one of ( ) gt, ( ) gt, (0 1 2) gt , or ( ) gt are preferentially parallel to (0 0 1) ms .…”

“…(1 0 0) ms 10.12 Å (1 0 0) ms 5.64 Å (1 0 0) ms 6.03 Å (1 0 0) ms 2.97 Å 6.06 Å (1 0 0) ms 3.23 Å (0 7 9) gt 9.56 Å (0 7 9) gt 5.39 Å (0 7 9) gt 4.99 Å (0 7 9) gt 5.39 Å (0 7 9) gt 3.22 Å ( − 7 7 9) gt 9.99 Å ( www.nature.com/scientificreports/ from (0 1 1) gt , respectively. This generally agrees with previous results [16][17][18]21 , suggesting that garnet nucleates epitaxially on the crystal structure of muscovite and/or biotite in the more exotic microstructures presented in those studies. Our results and interpretations further show that epitaxial nucleation is not restricted to unusual microstructures, but instead may be important during crystallization of many or most regional and contact metamorphic rocks.…”

“…Despite its cubic crystal structure and common form as a porphyroblast, garnet has been previously identified to crystallize via epitaxial and topotaxial relationships with muscovite, biotite, and pyroxene. This manifests as the parallelism of high symmetry crystal planes in one phase to those in another phase 15,16 with the most commonly reported relationship being {1 1 0} garnet // (0 0 1) mica (where '//' = 'parallel' and (hkl) are crystal planes) [16][17][18][19] , though others have also been identified [20][21][22] . However, these examples all either represent unusual microstructures, such as atoll or snowball garnets 21,22 , or formed at extreme environments such as ultra-high pressure metamorphism 17 .…”