Evidence for early Pliocene and late Miocene transgressions in southern Patagonia (Argentina): 87Sr/86Sr ages of the pectinid "Chlamys" actinodes (Sowerby)

Numerical ages based on 87Sr/86Sr dating of calcitic shells belonging to the pectinid "Chlamys" actinodes (Sowerby) document the only late Miocene (Tortonian) sea flooding event in the Austral Basin at Cabo Buentiempo (8.95±0.82Ma, 2 s.e.), and provide evidence of the first documented early Pliocene (Zanclean) transgression in Argentina recorded at Cañadón Darwin (5.15±0.18Ma, 2 s.e., Austral Basin) and at Terraces of Cerro Laciar (5.10±0.21Ma, 2 s.e.), southern San Jorge Basin). The sedimentary rocks deposited during the Tortonian are correlated with the youngest beds deposited by the "Entrerriense Sea" that covered northern Patagonia. The Zanclean marine episode is correlated with the long-term cycle represented in the Southern Hemisphere by the flooding events recorded in Cockburn and James Ross Islands (Antarctica) and in North-Central Chile.Facultad de Ciencias Naturales y Muse

Evidence for early Pliocene and late Miocene transgressions in southern Patagonia (Argentina): 87Sr/86Sr ages of the pectinid "Chlamys" actinodes (Sowerby)

Numerical ages based on 87Sr/86Sr dating of calcitic shells belonging to the pectinid "Chlamys" actinodes (Sowerby) document the only late Miocene (Tortonian) sea flooding event in the Austral Basin at Cabo Buentiempo (8.95±0.82Ma, 2 s.e.), and provide evidence of the first documented early Pliocene (Zanclean) transgression in Argentina recorded at Cañadón Darwin (5.15±0.18Ma, 2 s.e., Austral Basin) and at Terraces of Cerro Laciar (5.10±0.21Ma, 2 s.e.), southern San Jorge Basin). The sedimentary rocks deposited during the Tortonian are correlated with the youngest beds deposited by the "Entrerriense Sea" that covered northern Patagonia. The Zanclean marine episode is correlated with the long-term cycle represented in the Southern Hemisphere by the flooding events recorded in Cockburn and James Ross Islands (Antarctica) and in North-Central Chile.Facultad de Ciencias Naturales y Muse

Definition of Late Cretaceous stage boundaries in Antarctica using Sstrontium ssotope stratigraphy

New 87Sr/86Sr analyses of macrofossils from 13 key marker horizons on James Ross and Vega Islands, Antarctica, allow the integration of the Antarctic Late Cretaceous succession into the standard biostratigraphic zonation schemes of the Northern Hemisphere. The 87Sr/86Sr data enable Late Cretaceous stage boundaries to be physically located with accuracy for the first time in a composite Southern Hemisphere reference section and so make the area one of global importance for documenting Late Cretaceous biotic evolution, particularly radiation and extinction events. The 87Sr/86Sr values allow the stage boundaries of the Turonian/Coniacian, Coniacian/Santonian, Santonian/Campanian, and Campanian/Maastrichtian, as well as other levels, to be correlated with both the United Kingdom and United States. These correlations show that current stratigraphic ages in Antarctica are too young by as much as a stage. Immediate implications of our new ages include the fact that Inoceramus madagascariensis, a useful fossil for regional austral correlation, is shown to be Turonian (probably Late Turonian) in age; the “Mytiloides” africanus species complex is exclusively Late Coniacian in age; both Baculites bailyi and Inoceramus cf. expansus have a Late Coniacian/Early Santonian age range; an important heteromorph ammonite assemblage comprising species of Eubostrychoceras, Pseudoxybeloceras, Ainoceras, and Ryugasella is confirmed as ranging from latest Coniacian to very earliest Campanian. An important new early angiosperm flora is shown to be unequivocally Coniacian in age. Our strontium isotopic recalibration of ages strengthens the suggestion that inoceramid bivalves became extinct at southern high latitudes much earlier than they did in the Northern Hemisphere and provides confirmation that, in Antarctica, belemnites did not persist beyond the Early Maastrichtian

Origin of titanian pargasite in gabbroic rocks from the Northern Apennine ophiolites (Italy): insights into the late-magmatic evolution of a MOR-type intrusive sequence

Gabbroic rocks from Northern Apennine ophiolites contain accessory titanian pargasite, in interstices between plagioclase and clinopyroxene, and as rims around interstitial Fe-Ti-oxide phases. The origin of titanian pargasite has been evaluated by combining major, volatile and trace element microanalyses. Titanian pargasites show variable amounts of F and low Cl (0.03-0.23 and 640.03 wt%, respectively), and mg value ranging from 0.78 to 0.70 and from 0.61 to 0.53 in Mg- and Fe-rich rocks, respectively. Geothermometric calculations based on amphibole-plagioclase equilibrium yield temperatures of 900 \ub1 50\ub0C and 840 \ub1 50\ub0C for Mg- and Fe-rich rocks, respectively. Titanian pargasites are characterized by LREE depletion, nearly flat HREE and variable negative Eu anomalies. Total REE contents in titanian pargasites are higher and lower than in associated clinopyroxenes and apatites, respectively. In the chondrite-normalized patterns of titanian pargasites, Ba, K and Sr are markedly depleted relative to LREE, whereas Nb, Zr and Ti are slightly enriched to slightly depleted relative to neighboring REE. A separate of titanian pargasite from a Fe-rich gabbroic rock was analyzed for Sr isotopic composition; its initial 87Sr/86Sr falls within the range of modern N-MORB and is consistent with those of fresh Mg-rich gabbroic rocks. Titanian pargasite most likely formed by an igneous liquid with relatively high H2O content (4.4-6.1 wt%) and a slight LREE enrichment. The origin of this liquid has been ascribed to the percolation in the gabbroic crystal mush of a H2O-rich agent of igneous origin, WHICH could be a trondhjemite liquid or an exsolved fluid. Probably, such interaction triggered a post-cumulus crystallization process that finally yielded the precipitation of titanian pargasite

Adakites without slab melting: high pressure differentiation of island arc magma, Mindanao, the Philippines

New geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr / Y and depletion of the heavy rare earth elements. 87Sr / 86Sr and 143Nd / 144Nd ratios of the adakites do not support melting of the subducted Philippine Sea Plate but resemble Pliocene arc lavas generated in the same subduction zone. Excepting the heavy rare earth elements, the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge. The wide range of SiO2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature. (1) Adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet-bearing assemblage. (2) Solidified basaltic rock containing garnet melted to yield adakitic magma. In either case the basaltic precursor was generated from fluid-modified mantle then differentiated within the garnet stability field. In Surigao this requires differentiation within mantle. The Surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth. This has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks