P- and S-wave velocities of the lowermost crustal rocks from the Kohistan arc: Implications for seismic Moho discontinuity attributed to abundant garnet
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Summary This paper presents wet chemical analyses of forty-one pyroxenes (thirty-nine of them new) from pyroxene granulites and a few neighbouring rocks from Swat Kohistan and the adjoining Indus Valley. The granulites, considered to be derived from plutonic norites of an island arc tholeiitic nature, constitute one of the most extensive belts of its kind in the world. The pyroxenes are not unusual in any respect but they bear a closer resemblance to metamorphic than to igneous pyroxenes crystallized from deep-seated tholeiitic magmas. The distribution of Mg, Fe, and Mn (average K D Mg−Fe =0.57) and the tie-line intersections on the Wo-En join (generally from Wo 80.5 to Wo 76.5 ) between the coexisting eighteen pyroxene pairs from the granulites are discussed. Based on eight different methods of geothermometry and other considerations, it is concluded that the pyroxene granulites were metamorphosed at around 800 ° C and 7 to 8 kbar.
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The recrystallization of a tholeiitic basalt to garnet granulite and to eclogite has been experimentally examined at temperatures ranging from 800°C up to the solidus of the rock. In addition, the solidus of tholeiitic basalt has been redetermined. The slope of the boundary separating the field of garnet granulite frpm that of basalt is P kb = 0.014 T°C - 5.4 kb. The boundary separating the field of garnet granulite from the field of eclogite has the equation P kb = 0.020 T°C + 4.0 kb. The equation of the newly determined solidus of tholeiitic basalt is T = 6.0 P kb + 1105°C. Curves relating the densities of the mineral assemblages to temperatures and pressures of formation are shown. One relatively abrupt change in density occurs as basalt recrystallizes to garnet granulite, and a second relatively sharp density increase occurs as garnet granulite is recrystallized to eclogite.
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Abstract High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 ) system, a series of P – T – X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low X Al [Al 2 O 3 /(Na 2 O + CaO + K 2 O + FeO + MgO + Al 2 O 3 ) < 0.35, mole proportion] or high X Mg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the X Al values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.
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