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Source characteristics of the basement rocks from the Sulu and Celebes Basins (Western Pacific): chemical and isotopic evidence
No full textGeochemistry of garnet pyroxenite layers from the External Liguride mantle peridotites (Northern Apennine, Italy): evidence for recycling of crustal material in a ophiolitic mantle sequence
No full textPetrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism.
No full textTrace element and Nd-O isotope compositions of garnet pyroxenite layers from the External Liguride peridotites (Northern Apennine ophiolites): evidence for recycling of mafic continental crust?
No full textPetrology of predazzo magmatic complex (Trento, Italy)
No full textAn extensive study of the Triassic volcano-plutonic complex of Predazzo has been carried out. As it is known, Predazzo intrusion represent the main plutonic body of the magmatic activity which affect the South Alpine domain during the Triassic age. It is composed by a ring-shaped small intrusive bodies emplaced between 238 and 232 My (Laurenzi et al., 1997), partly coeval with the volcanic products which overlye the complex.
Petrographyc and geochemical features, together with age relationships allow to depict a rather complicated tectono-magmatic evolution, resulting in several small magma pulses with different magmatic affinity injected in a short time gap.
According to Visonà (1997) four main sequences were distinguished on the basis of silica saturation degree, mineral phases and compositions. They are:
- a shoshonitic silica saturated series (M1),
- a shoshonitic silica oversaturated series (M2),
- a potassic-alkaline series (M3) and
- a probably calc-alkaline series made up only by the granites (G).
M1 represents the main part of the outer ring of the complex and is mainly constituted by monzo-gabbros and monzonites, with subordinate gabbros and pyroxenites and syenites. Diopsidic to augitic clinopiroxene is the most common phase, followed by olivine, Mg-horneblende and minor ortopiroxene, while plagioclase is the most abundant sialic phase, followed by K-feldspar and scarse quartz. It represents the most common series in the Dolomitic area as both intrusives (M.ti Monzoni, Cima Pape) and volcanics.
M2 is mainly represented by monzo-diorites, with minor quartz-monzonites and quartz-syenites. Clinopiroxene and amphibole, with similar composition to those of M1, are common; quartz is more abundant in monzo-diorites, monzonites and syenites, coherently with he marked silica-oversaturation of the series.
M3 outcrops exclusively in the eastern part of the complex (Val di Viezzena). It is characterized by abundance of differentiated products (syenites), with subordinate monzo-gabbros and monzonites, all bearing nepheline. In this series clinopiroxene is salitic to aegirinaugitic in composition and amphibole varies from Mg-hastingsite to Fe-pargasite. The presence of titanite, apatite and garnet (rare) is peculiar.
Calc-alkaline granites constitute the core of the complex. An “I Type” nature was proposed by Visonà, (1997) for these rocks, considered differentiated from calc-alkaline basic magmas; the presence of muscovite is related to secondary deuteric processes overimposed on primary biotite crystals.
Differentiation processes were modelled using a mass-balance calculations. Results indicate that crystallization played a dominant role in the differentiation of the three series.
On the basis of field relationship between the various lithotypes the temporal succession of the intrusion may be put forward:
M1 represent the oldest intrusion in the western and north-western part of the complex. It was followed by M2 in the eastern and south-eastern part. A gradational contact between lavas, sills and the M1 and M2 intrusives suggests concomitant, or slightly older, volcanic episodes. The youngest intrusive event is represent by the granitic rocks, which are clearly intruded in M1 and M2 as well as in the volcanics. Granites, in turn, are cut by several K-lamprophyric dykes, which might be analogous to the M3 parental magmas. The timing of M3 intrusion remain still uncertain, since it cuts M1 and M2, but no relationship were found with the granites
Structure and shearing conditions in the Day Nui Con Voi massif: Implications for the evolution of the Red River shear zone in northern Vietnam
Get PDFThe Day Nui Con Voi massif bears a record of the Red River shear zone (RRSZ) activity in North Vietnam. It forms a large-scale antiformal "core complex"-type structure, bounded by the Song Hong and Song Chay faults. The kinematics of both faults are identical and reflect transtensional shear initiated under upper amphibolite facies conditions and propagated into greenschist facies. Microfabric analysis establishes that both extensional and strike-slip shearing initiated between 700 and 500°C. The RRSZ evolved from a single, subvertical fault, which, due to strike-perpendicular extension, underwent progressive dilation. The created space was "intruded" by already metamorphosed and deformed ductile middle crust in the form of a gneissic "dome." Both strike-slip and extensional shearing were accommodated in the limbs of the antiform, while its core was uplifted from midcrustal level bearing only a minor record of sinistral shear. Copyright 2007 by the American Geophysical Union
Source characteristics of the basement rocks from the Sulu and Celebes Basins (Western Pacific): Chemical and isotopic evidence
No full textNew Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacific, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between 87Sr/86Sr and 143Nd/144Nd show that the seafloor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORB), and distinct from East Pacific Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics are different, in that they have distinctly lower ɛNd(T) for a given ɛSr(T), compared to Sulu and Celebes basalts. In the 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high Δ7/4 Pb (5.3–9.3) and D8/4 Pb (46.3–68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type (“normal”) mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low Nd(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western Pacific region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources
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