Petrology of Felsic Granulites, Metapelites, Metabasics, Ultramafics, and Metacarbonates from Southern Calabria (Italy): Prograde Metamorphism, Uplift and Cooling of a Former Lower Crust

A high-grade metamorphic terrane in the southern part of theCalabrian massif (South Italy) has been petrographically mappedand the dominant rock types petrologically investigated. Bothmethods of investigation have led to the recognition of a continuoussection through a former lower crust which is 7 km thick. Itslower part consists predominantly of metabasic rocks togetherwith minor felsic granulites, its upper part of metapeliteswith minor metabasic and metacarbonate rocks. The rocks experienced a common two-stage prograde metamorphicevolution in which the second stage occurred after the lastpenetrative deformation. The prograde metamorphism which, accordingto radiometric dates, ended in late Hercynian time, was of themedium-pressure type of Miyashiro (1961), and equilibrationoccurred in the ‘medium-pressure granulite field’(characterized by the instability of olivine-plagioclase aswell as garnet-clinopyroxene-quartz). Estimates of the highest_P—_T conditions of prograde metamorphism give 7–8kb and approximately 800°C at the base, but 5–6 kband 650–700°C at the top of the section, at whichthe paragenesis staurolite-quartz indicates the transition tothe amphibolite facies. The existence of a metamorphic gradientin the lower crust section is demonstrated by the systematicchange in the compositions of ferro-magnesian minerals in divariantmetapelitic assemblages. The metamorphic evolution during the excavation history of theformer lower crust has been reconstructed using the numerousdisequilibrium reaction textures preserved in most rock types.The highest metamorphic conditions ended with a pressure decreaseof approximately 1.5 to 2 kb, which was followed by a periodof quasi-isobaric cooling in the middle crust. During this cooling,the stability field of the ‘high-pressure granulites’(garnet-clinopyroxene-quartz) was reached. The pressure decrease, which induced the end of the high-temperaturehistory of the lower crust, is interpreted as reflecting theerosion of the uppermost crustal levels as a response to overlappingof large crustal segments during the Hercynian orogeny. Consequently,the deduced P—T path of the upper, i.e. overthrust crustalsegment is thought to have been tectonically controlled.     

Petrology of Whiteschists and Associated Rocks at Mautia Hill (Tanzania): Fluid Infiltration during High-Grade Metamorphism?

Talc–kyanite schists (whiteschists), magnesiohornblende–kyanite–talc–quartzschists and enstatite–sapphirine–chlorite schistsoccur at Mautia Hill in the East African Orogen of Tanzania.They are associated with metapelites and garnet–clinopyroxene–quartzmetabasites. Geobarometry (GASP/GADS equilibria) applied tothe latter two rock types indicates a peak pressure of P = 10–11kbar. These results are confirmed by the high fO₂ assemblagehollandite–kyanite–quartz and late-stage manganianandalusite that contains up to 19·5 mol. % Mn₂SiO₅. Maximumtemperatures of T = 720°C are inferred from late-stage yoderite+ quartz. A clockwise P–T evolution is constrained byprograde kyanite inclusions in metapelitic garnet and late-stagereaction rims of cordierite between green yoderite and talcthat reflect conditions at least 3–4 kbar below the peakpressure. Oxidizing conditions are recorded throughout the metamorphichistory of the whiteschists and chlorite schists, as indicatedby the presence of haematite coexisting with pseudobrookiteand/or rutile. Increasing water activity near peak pressuresis thought to have led to the breakdown of the high-pressureassemblages (Tlc–Ky–Hem and Mg-Hbl–Ky–Hem)and the subsequent formation of certain uncommon minerals, e.g.yellow sapphirine, Mn–andalusite, green and purple yoderite,piemontite and boron-free kornerupine. The proposed increasein water activity is attributed to fluid infiltration resultingfrom the devolatilization of underlying sediments during metamorphism. KEY WORDS: fluid infiltration; high-pressure amphibolite facies; East African Orogen; Pan-African; whiteschist     

The Petrology of Basanite-Tephrite Intrusions in the Erongo Complex and Implications for a Plume Origin of Cretaceous Alkaline Complexes in Namibia

Basanite intrusions from the Early Cretaceous Erongo complex,Namibia, have compositions consistent with near-primary mantlemelts derived from a depth of at least 100 km. These rocks providea key reference for the mantle component(s) involved in breakup-relatedmagmatism in this region. Initial Sr–Nd–Pb isotoperatios of the Erongo basanites and associated tephrites andphonotephrites (⁸⁷Sr/⁸⁶Sr = 0·70425–0·70465;