Early Miocene Antarctic glacial history: New insights from heavy mineral analysis from ANDRILL AND–2A drill core sediments

The present study deals with heavy mineral analysis of late Early Miocene marine sediments recovered in the McMurdo Sound region (Ross Sea, Antarctica) during the ANDRILL— SMS Project in 2007. The main objective is to investigate how heavy mineral assemblages reflect different source rocks and hence different provenance areas. These data contribute to a better understanding of East Antarctica ice dynamics in the Ross Sea sector during the Early Miocene (17.6–20.2 Ma), a time of long-term global warming and sea level rise. The AND-2A drill core recovered several stratigraphic intervals that span from Early Miocene to Pleistocene and it collected a variety of terrigenous lithologies. The heavy mineral assemblages of the lower 650-m-thick sedimentary succession were analyzed through SEM observations and SEM–EDS microanalyses on heavy mineral grains. The heavy mineral analysis shows that the sediments are a mix of detritus dominated by McMurdo Volcanic Group sources most likely located in the present-day Mount Morning area (Proto-Mount Morning) with minor contribution from Transantarctic Mountains source rocks located west of the drill site. The heavy mineral assemblages in Interval 1 indicate that between 20.2 and 20.1 Ma, the grounding line of the ice sheet advanced to a position near the present-day Mount Morning volcanic center. During deposition of Interval 2 (20.1–19.3 Ma), the ice sheet most likely experienced a dynamic behavior with interval of ice advance alternating with periods of ice retreat, while Interval 3 (19.3–18.7 Ma) records further retreat to open water conditions. A dynamic behavior is noted in Interval 4 (18.7–17.6 Ma) with a decreasing contribution of materials derived from the basalts of the Mount Morning volcanic center located to the south of the drill site and a consequent increasing contribution of materials derived from the Transantarctic Mountains to the west of the drill site

Li,B-rich raethian metabauxite, Tuscany, Italy: reworking of older bauxites and igneous rocks

3noreservedLi, B-rich metabauxite layers occur in two outcrops of talus metabreccias intercalated between Norian and Liassic marbles in the Alpi Apuane, Tuscany, Italy. Field evidence, petrographical, geochemical, and paleogeographic, paleoclimatologic considerations suggest that bauxitic material was derived from a multistage reworking of Late Carboniferous-early Autunian primary deposits now completely removed. The identified lithotypes are: cookeite-rich metabauxite, cookeitebearing metabreccia, chloritoid schist and chloritoid phyllite. The Mg content of calcite, and the Si content of muscovite and mineral assemblages indicate temperatures of 350°-380°C, pressures of < 7 kbar and fo2-values of 10-30 to 10-24 for the Alpine metamorphism. In comparison with the average Mesozoic bauxite of Bronevoy et al. (1985) the metabauxite is: strongly enriched in Si, K, Ca, Mg, Li, B, Be, Ni and Ce; it shows comparable contents of Co, La, Eu, Tb, Ta and Th; it is depleted in Ga, Cr, V, Zn, Mn, Hf, U; and is strongly depleted in Sr, F, Cu. A metabauxite layer shows decreasing abundance of Fe and Ga from bottom to top with a pattern opposite to that of primary bauxite deposits (inverted profile). The strong enrichment of Li and B is not related to evaporitic conditions, since the MgO, Na2O and Sr contents are too low, and indicator minerals such as dravitic tourmaline, albite and scapolite are missing. During the Permian, volcanic products similar to the coeval peralkaline-peraluminous rocks of Corsica-Sardinia were widespread in Tuscany. B, Li-rich minerals were produced by circulation of hot hydrothermal fluids within the Permian volcanic rocks during and soon after their emplacement. The erosion of such hydrothermal products supplied detrital material to the talus metabreccia. © 1998 Elsevier Science B.V.mixedFRANCESCHELLI, M. ; TURBANTI, I.; Puxeddu, M.Franceschelli, M.; Turbanti, I.; Puxeddu, M

SEM and HRTEM study of Chlorite-Paragonite-Muscovite intergrowhths in porphyroclasts of a low grade metamorphic quarzite from verrucano formation (Northern Appenines, Italy)

Reseña de la comunicación presentada en el 16th General Meeting of the International Mineralogical Association (Pisa, Italia, 4-9 septiembre 1994)Low grade metamorphosed clastic rocks from Triassic Verrucano Formation (Northern Appenines, Italy) contained detrital and metamorphic minerals along with porphyroclasts of intergrown phyllosilicates: differences in chemical and mineralogical compositions between the clasts and the matrix can be sometime...Peer reviewe

Origin of intergrown phyllosilicate stacks from Verrucano metasediments (northern Apennines, Italy): A transmission and analytical electron microscopy study

3 páginas. Resumen de la comunicación presentada en el Spanish-Italian Meeting on Clay Minerals (Granada, Spain, 19-21 septiembre 1996)Grains of intergrown phyllosilicates (IPG) widely occur in cleaved and low-grade metamorphosed clastic rocks from Verrucano Group (northem Apennines, Italy) These stacks have been re-examined after Franceschelli et all(1989; 1991) using SEM and HRTEM in order to better characterize them and to clarify their origin.This work was financially supported by Italian Ministry of University and Scientific and Technological Research grants (MURST to I.M.) and by Spanish Research Project grants PB92-0961, PB92-0960 and Research Group 4065 of the Junta de AndaluciaPeer reviewe

Microstructures of intergrown phyllosilicate grains from Verrucano metasediments (northern Apennines, Italy)

12 páginas, 11 figuras, 3 tablasInterleaved phyllosilicate grains (IPG) of various compositions are widespread in low-grade Verrucano metasediments of the northern Apennines (Italy). They are ellipsoidal or barrel shaped, up to 300–400 μm long and they are often kinked and folded; phyllosilicate packets occur as continuous lamellae or as wedge-shaped layers terminating inside the grain. Using electron microscopy techniques (SEM, TEM) six types of IPG have been distinguished on the basis of their mineralogical composition: (1) Chl+Ms ± Kln; (2) Chl+ Ms+Pg ± Kln; (3) Ms+Prl ± Pg; (4) Ms+Prl+Su; (5) Ms+Prl+Chl+Su; (6) Su+Ms. Types (1) and (2) are mainly composed of chlorite, with Ms and Pg as minor phases; Kln grows on Ms in highly weathered samples. Types (3), (4), (5), and (6) are composed of muscovite, with intergrown Prl, Chl, Su and new-formed muscovite. The IPG show all kinds of contacts: from coherent grain boundaries with parallel basal planes and along-layer transitions to low- and high-angle grain boundaries. The IPG formed on pristine minerals such as chlorite and muscovite. The transformations took place during the prograde and retrograde metamorphic path of the rocks: they were facilitated by deformation and they occurred in equilibrium with a fluid phase, which allowed cation diffusion. Prograde reactions [Chl = Ms (or Pg); Ms = Prl; Ms = Chl] involve dehydration and sometimes a decrease in volume, whereas retrograde reactions (Ms = Kln; Ms = Su) involve hydration and an increase in volume. These transformations do not simply occur through an interchange of cations, but often involve deep structural changes: transitions from one phyllosilicate to another generally proceed through dissolution-recrystallization reactions. In conclusion, Verrucano IPG represent microstructural sites which have not completely equilibrated with the whole rock and whose mineral assemblage depends on the original composition of the microstructural sites.Financial support was supplied by the Italian Ministry of University and Scientific and Technological Research grants (MURST to I.M.) and Research Projects nº PB92-0961 and PB92-0960 of the Spain Ministry of Education as well as Research Group 4065 of the Junta de AndaluciaPeer reviewe

Short-range mobilization of elements during thermal metamorphism of the Kharlovo gabbro aureole

3sireservedMass balance and compositional variation in the outer part of the contact aureole shows that mass transfer of the main petrogenetic components was confined to small domains of a few hundredths of mm3 in chlorite-biotite-mica and epidote-plagioclase-titanomagnetite aggregates. The chemical homogeneity of the environment was practically unaffected by the thermal metamorphism. -AuthorsmixedLIKHANOV, I. ; REVERDATTO, V.; TURBANTI, I.Likhanov, I.; Reverdatto, V.; Turbanti, I

Metamorphic evolution of Verrucano metasediments in northern Apennines: new petrological constrains

14 páginas, 9 figuras, 3 tablasThe Triassic Verrucano metasediments largely crop out along the mid Tuscan ridge, in the northern Apennines. The metasediments mainly consist of ferriferous quartzites and Al-rich metapelites and contain a specific mineralogy which lead to the reconstruction of their P-T path. The newly-formed metamorphic minerals are syn-to post-kinematics and they consist of magnesiocarpholite, muscovite, chlorite, paragonite, pyrophyllite, sudoite, kaolinite, and chloritoid, Magnesiocarpholite locally occurs only in a few syn-metamorphic quartz veins. Microanalytical techniques (SEM-EDS, TEM) allowed to collect compositional and microstructural information at the micron scale, to recognize the actual mineral reactions and their relative chronology. For Verrucano quartzites the stable paragenesis is chloritoid-chlorite-pyrophyllite, with sudoite and kaolinite being incompatible AFM phases. Thermodynamic calculations for Verrucano quartzites indicates that these rocks experienced a) relative high P/low T metamorphism (peak conditions ranging from 350 degrees C at 8 kbar to 420 degrees C at 10 kbar); b) a retrograde path characterized by cooling during decompression. These data demonstrate the usefulness of metapelites in deciphering the conditions of very low grade metamorphism. Besides, they support the regional geodynamic evolution which outlines the subduction of the continental crust of the Adria microplate under the European plate (Oligocene).Peer reviewe