Oman Metamorphic Sole

This album shows microstructures in the garnet-clinopyroxene amphibolites from Wadi Sumeini, immediately beneath the Semail ophiolite, and gradually builds up the case that these were originally metabasic Grt-Cpx granulites before being hydrated, sheared and accreted onto the base of the ophiolite mantle section. Petrography by Dave Waters, October 2011, using samples from the collection of Mike Searle and Jon Cox.
OM54-i17  Published accounts stress the tectonic fabric parallel to the contact with, and fabric within, the base of the harzburgites. OM54, ppl, x2 obj. OM54-i18  Published accounts stress the tectonic fabric parallel to the contact with, and fabric within, the base of the harzburgites. OM54, xpol, x2 obj. OM58-i35  Dominant fabric elements are a narrow compositional banding of Hbl-rich and Pl-rich layers, and a shape fabric in lozenge-shaped hornblende grains. OM58, ppl, x2 obj. OM58-i36  Dominant fabric elements are a narrow compositional banding of Hbl-rich and Pl-rich layers, and a shape fabric in lozenge-shaped hornblende grains. OM58, ppl, x4 obj. OM65-i1  More irregularly shaped Grt and Cpx are scattered in this fabric. OM65, ppl, x2 obj.
OM65-i2  More irregularly shaped Grt and Cpx are scattered in this fabric. OM65, xpol, x2 obj. OM58-i30  Coarser grains commonly occur in lenticular augen; brown hornblende in this case. OM58, ppl, x2 obj. OM58-i31  Coarser grains commonly occur in lenticular augen; brown hornblende in this case. Inclusion in Hbl is prehnite possibly replacing Pl. OM58, xpol, x2 obj. OM64-i93  Coarser grains commonly occur in lenticular augen; here is Cpx, with a paler, embayed margin. OM64, ppl, x4 obj. OM64-i94  Coarser grains commonly occur in lenticular augen; here is Cpx, with an embayed margin showing higher birefringence. OM64, xpol, x4 obj.
OM52-i11  Here is another large Cpx grain with inclusions of plagioclase and minor replacement by hornblende at its margins. OM52, ppl, x2 obj. OM52-i12  This large Cpx shows what may be exsolution blebs, although attempts to analyse these imply amphibole compositions. OM52, xpol, x2 obj. OM58-i32  In practice, most larger Cpx occurrences are granoblastic aggregates. OM58, ppl, x2 obj. OM58-i34  In practice, most larger Cpx occurrences are granoblastic aggregates. OM58, xpol + mica plate, x2 obj. OM60-i52  In practice, most larger Cpx occurrences are granoblastic aggregates. OM60, ppl, x2 obj.
OM60-i53  In practice, most larger Cpx occurrences are granoblastic aggregates. Note apparent exsolution in largest grain. OM60, xpol, x2 obj. OM64-i76  In practice, most larger Cpx occurrences are granoblastic aggregates, here with interstitial hornblende. OM64, ppl, x4 obj. OM64-i77  In practice, most larger Cpx occurrences are granoblastic aggregates, here with interstitial hornblende. OM64, xpol, x4 obj. OM54-i15  In practice, most larger Cpx occurrences are granoblastic aggregates, here in a very broad lens. OM54, ppl, x2 obj. OM54-i16  In practice, most larger Cpx occurrences are granoblastic aggregates, here in a very broad lens. OM54, xpol, x2 obj.
OM64-i88  Some larger individual Cpx crystals are deformed by lattice bending. OM64, ppl, x2 obj. OM64-i87  Some larger individual Cpx crystals are deformed by lattice bending. OM64, xpol, x2 obj. OM60-i61  Larger Cpx grains very commonly show what appear to be exsolution lamellae, suggesting a very high T early history. OM60, ppl, x4 obj. OM60-i62  Larger Cpx grains very commonly show what appear to be exsolution lamellae, suggesting a very high T early history. OM60, xpol, x4 obj. OM60-i54  Garnets occur in several apparent forms, here in small anhedral grains. OM60, ppl, x2 obj.
OM60-i55  Garnets occur in several apparent forms, here in small anhedral grains. OM60, xpol, x2 obj. OM61-i73  But even these small grains have large inclusions or are intergrown with, for example, Cpx. OM61, ppl, x2 obj. OM61-i74  But even these small grains have large inclusions or are intergrown with, for example, Cpx. OM61, xpol, x2 obj. OM65-i6  Cpx is commonly found as relatively large subrounded inclusions in Grt. OM65, ppl, x2 obj. OM65-i7  Cpx is commonly found as relatively large subrounded inclusions in Grt. OM65, ppl, x2 obj.
OM64-i78  Cpx is commonly found as relatively large subrounded inclusions in Grt. OM64,ppl, x2 obj. OM64-i79  Cpx is commonly found as relatively large subrounded inclusions in Grt. OM64, xpol, x2 obj. OM58-i26  This Grt-Cpx association is an intergrowth, with several parts of the Cpx in optical continuity betwen "inclusions". OM58, ppl, x2 obj. OM58-i27  This Grt-Cpx association is an intergrowth, with several parts of the Cpx in optical continuity betwen "inclusions". OM58, ppl, x2 obj. OM64-i83  This Grt-Cpx association also shows some optical continuity betwen Cpx "inclusions". OM64, xpol, x4 obj.
OM64-i85  This Grt-Cpx association also shows some optical continuity betwen Cpx "inclusions", perhaps more obvious when rotated and with sensitive tint plate. OM64, xpol, x4 obj. OM61-i70  Larger garnets form lenticular enclaves wrapped by the fabric. Cpx is very commonly associated with Grt in these enclaves. OM61, ppl, x2 obj. OM60-i56  The Cpx associated with Grt here also contains Hbl inclusions (not a reaction rim, so quite possibly in peak equilibrium). OM60, ppl, x2 obj. OM60-i58  The Cpx associated with Grt here also contains Hbl inclusions (not a reaction rim, so quite possibly in peak equilibrium). OM60, xpol+sens.tint, x2 obj. OM61-i67  Many other lenticular domains are more obviously rock associations rather than merely grains with inclusions. OM61, ppl, x2 obj.
OM61-i69  Many other lenticular domains are more obviously rock associations rather than merely grains with inclusions. OM61, xpol+sens.tint, x2 obj. OM59-i43  Many other lenticular domains are more obviously rock associations rather than merely grains with inclusions. This one contains Hbl, possibly an alteration product of Cpx, or possibly primary. OM59, ppl, x2 obj. OM59-i44  Many other lenticular domains are more obviously rock associations rather than merely grains with inclusions. This one contains Hbl, possibly an alteration product of Cpx, or possibly primary. OM59, ppl, x2 obj. OM59-i45  Many other lenticular domains are more obviously rock associations rather than merely grains with inclusions. This one contains Hbl, possibly an alteration product of Cpx, or possibly primary. OM59, xpol+mica plate, x2 obj. OM64-i89  Here are more suggestions that brown Hbl and plagioclase may be part of the primary enclave assemblage. OM64, ppl, x4 obj.
OM64-i91  There are more suggestions that brown Hbl and plagioclase may be part of the primary enclave assemblage. OM64, xpol+mica plate, x4 obj. OM59-i39  In some rocks the enclave Grt-Cpx domains are even more obviously oval porphyroclasts in a foliated Hbl-dominated matrix. OM59, ppl, x2 obj. OM59-i41  In some rocks the enclave Grt-Cpx domains are even more obviously oval porphyroclasts in a foliated Hbl-dominated matrix. OM59, xpol+mica plate, x2 obj. OM61-i63  In some rocks the enclave domains are even more obviously oval porphyroclasts in a foliated Hbl-dominated matrix. Here the enclave assemblage is Grt-Cpx-Pl-Ilm-[Hbl]. OM61, ppl, x2 obj. OM61-i64  In other words - the protolith of the high-T sole garnet amphibolites is a well-annealed Grt-Cpx granulite, comparable in assemblage and peak P-T conditions to the highest-grade (Badcallian) Lewisian metabasites. OM61, xpol, x2 obj.
OM61-i65  The sole precursor had acquired this state before being accreted and sheared at the basal contact of the harzburgites. The shearing that distributed these enclaves in the Hbl-dominated fabric occurred under more hydrous conditions, but still at very high T (probably >750°C). OM61, xpol+mica plate, x2 obj. OM61-i66  For further details of the P-T evolution, and integration with the field evidence and tectonic setting, see Cowan et al. (2014), Structure of the metamorphic sole to the Oman Ophiolite, Sumeini Window and Wadi Tayyin: implications for ophiolite obduction processes, in: Rollinson et al., (eds) Tectonic Evolution of the Oman Mountains. Geological Society, London, Special Publications, 392, 155–175. http://dx.doi.org/10.1144/SP392.8