Melt-rock interaction in the lower crust based on silicate melt inclusions in mafic garnet granulite xenoliths, Bakony–Balaton Highland

Major and trace element composition of silicate melt inclusions (SMI) and their rock-forming minerals were studied in mafic garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field (Western-Hungary). Primary SMIs occur in clinopyroxene and plagioclase in the plagioclase-rich domains of mafic garnet granulites and in ilmenite in the vicinity of these domains in the wall rock. Based on major and trace elements, we demonstrated that the SMIs have no connection with the xenolith-hosting alkaline basalt as they have rhyodacitic composition with a distinct REE pattern, negative Sr anomaly, and HFSE depletion. The trace element characteristics suggest that the clinopyroxene hosted SMIs are the closest representation of the original melt percolated in the lower crust. In contrast, the plagioclase and ilmenite hosted SMIs are products of interaction between the silicic melt and the wall rock garnet granulite. A further product of this interaction is the clinopyroxene–ilmenite±plagioclase symplectite. Textural observations and mass ­balance calculations reveal that the reaction between titanite and the silicate melt led to the formation of these assemblages. We propose that a tectonic mélange of metapelites and (MOR-related) metabasalts partially melted at 0.3–0.5 GPa to form a dacitic–rhyodacitic melt leaving behind a garnet-free, plagioclase+clinopyroxene+orthopyroxene+ilmenite residuum. The composition of the SMIs (both major and trace elements) is similar to those from the middle Miocene calc-alkaline magmas, widely known from the northern Pannonian Basin (Börzsöny and Visegrád Mts., Cserhát and Mátra volcanic areas and Central Slovakian VF), but the SMIs are probably the result of a later, local process. The study of these SMIs also highlights how crustal contamination changes magma compositions during asthenospheric Miocene ascent.

Gabbronorite from Jijal Complex, Kamila Amphibolite Belt and Chilas Complex, Northern Pakistan: Implications for Arc Genesis

Rocks of gabbronoritic composition occur in three principal tectono-stratigraphic units forming the lower andmiddle parts of the Kohistan Island arc (KIA). These include the Jijal complex (JC), the Kamila Amphibolite belt (KAB)and the Chilas complex (CHC). The Jijal complex constitutes the lowermost part and hence is regarded as the root zoneof KIA. Its north-eastern part adjacent to KAB contains gabbronorite as a minor component in the form of small irregularpatches and layers within garnet granulite. The JC gabbronorite is sub-equigranular, medium to coarse grained, largelymassive and consists of variable amounts of plagioclase (53-71 %), orthopyroxene (14-27 %) and clinopyroxene (11-19%) as essential constituents and accessory to minor amounts of amphibole (1-9 %), opaque ore (1-6 %) and orthoclase(1-4 %). The occurrence and distribution of biotite, epidote, chlorite, clay, sericite, muscovite, quartz and actinolite inthe studied samples suggest their formation through alteration and/ or reaction between pre-existing minerals. In manycases, these minerals are disposed such that a variety of simple and complex corona structures are produced. The principalpetrographic features (modal composition, optical properties of the major mineral phases, exsolution in pyroxenes,products of alteration and reactions and the resulting corona textures) of the JC gabbronorite are broadly similar togabbronorites from both the KAB and CHC. Although the observed similarities could reflect identical physico-chemicalconditions during subsolidus or metamorphic re-equilibration, the possibility of a genetic relationship amonggabbronorites from all the three tectono-magmatic units of the KIA (i.e. the JC, KAB and CHC) cannot be ruled out.

Eclogite and Garnet Pyroxenite Xenoliths from Kimberlites Emplaced Along the Southern Margin of the Kaapvaal Craton, Southern Africa: Mantle or Lower Crustal Fragments?

Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000 °C, at pressures of 1·7 ± 0·4 GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55 km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835 °C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2 Ga.

Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the Araçuaí Orogen in southern Bahia, including a discussion on the tectonic meaning of the current results

In southern Bahia, there are outcrops of migmatites and granulites in the Jequitinhonha Complex, which is part of the northern portion of the Araçuaí Orogen. Migmatites (garnet-cordierite diatexite) dominate the metamorphic rocks and host lenses and layers of felsic garnet granulite. The conditions of temperature and pressure of metamorphism were calculated using conventional thermobarometry and the software THERMOCALC. Values around 850 °C and 7 kbar were obtained with THERMOCALC. The calculations for the garnet-cordierite diatexite were made considering aH2O equal to 1, but the best results of calculations for the granulites are obtained with aH2O values of 0.3. Pressure values obtained with GAPES resulted in consistent values with THERMOCALC, but the pair garnet-orthopyroxene always produces low values for temperature and high ones for pressure. The results are consistent with the presence of the pair garnet and cordierite in diatexite and orthopyroxene in felsic granulite. From the tectonic point of view, the setting in which metamorphism of these rocks occurred requires high heat flow with a thermal anomaly in mid continental crust, as indicated by values of 7 kbar. Recent studies have favored the closure of a back-arc basin for this tectonic setting, but it does not solve the problem that the time span between metamorphic peak and the end of granite intrusions, involving large bodies of charnockite, is more than 80 million years. The model of tectonic switching is suggested here as it can explain the maintenance of high temperatures for a more extended interval of time.