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Strain coupling between upper mantle and lower crust: natural example from the Běstvina granulite body, Bohemian Massif
Author(s) -
MACHEK M.,
ULRICH S.,
JANOUŠEK V.
Publication year - 2009
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2009.00830.x
Subject(s) - granulite , geology , peridotite , massif , geochemistry , olivine , mantle (geology) , crust , layering , eclogite , lineation , petrology , facies , seismology , tectonics , subduction , geomorphology , botany , structural basin , biology
Strain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spačice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S 1 fabric that is superposed on the S 0 fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S 0 fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S 1 in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism < c > slip in quartz respectively. While the S 1 fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S 1 fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S 0 fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S 1 fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spačice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex ( c . 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.