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1.
International Journal of Earth Sciences - The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to...  相似文献   
2.
A deformed ca. 570 Ma syenite–carbonatite body is reported from a Grenville-age (1.0–1.2 Ga) terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. This is the first recognition of such a rock assemblage in the basement of the Central Andes. The two main lithologies are coarse-grained syenite (often nepheline-bearing) and enclave-rich fine-grained foliated biotite–calcite carbonatite. Samples of carbonatite and syenite yield an imprecise whole rock Rb–Sr isochron age of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029); SHRIMP U–Pb spot analysis of syenite zircons shows a total range of 206Pb–238U ages between 433 and 612 Ma, with a prominent peak at 560–580 Ma defined by homogeneous zircon areas. Textural interpretation of the zircon data, combined with the constraint of the Rb–Sr data suggest that the carbonatite complex formed at ca. 570 Ma. Further disturbance of the U–Pb system took place at 525 ± 7 Ma (Pampean orogeny) and at ca. 430–440 Ma (Famatinian orogeny) and it is concluded that the Western Sierras Pampeanas basement was joined to Gondwana during both events. Highly unradiogenic 87Sr/86Sr values in calcites (0.70275–0.70305) provide a close estimate for the initial Sr isotope composition of the carbonatite magma. Sm–Nd data yield Nd570 values of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean from depleted mantle with a component from Meso/Neo-proterozoic lower continental crust.  相似文献   
3.
The age of the granites of the La Esperanza region of the Somuncura Batholith, North Patagonian Massif has been revised, based in part on Rb---Sr whole-rock data obtained by reanalyzing samples from a previous study. The new ages are 258 ± 15 Ma for the Prieto Granodiorite and 259 ± 16 Ma for the Donosa Granite, both from the older La Esperanza plutonic complex, and 239 ± 4 Ma for the Calvo Granite, from the younger volcano-plutonic Dos Lomas complex. The initial 87Sr/86Sr ratios are all in the range 0.7070–0.7076. The ages probably correspond stratigraphically to Late Permian and Early Triassic for the two complexes, respectively, consistent with traditional geologic interpretation. Together with recently published Triassic ages from the Batholith of Central Patagonia, it is clear that the acidic volcano-plutonic associations of northern Patagonia are very latest Paleozoic and Mesozoic in age. They are not obviously related to terrane collision but are part of a Permo-Triassic acid magmatic province that extends throughout the central Andes and that preceded, or was associated with, the early rifting of Gondwana.  相似文献   
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6.
New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullún), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ullún. The oldest rocks are the Andino-type granitic orthogneisses of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma. The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites (1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The emplacement of these mantle-derived magmas during an extensional episode produced a widespread thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and back-arc complexes dominated the Sierra de Pie de Palo–Ullún sector, that was fully developed ca. 1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at ～1165 Ma was roughly coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are registered in the Pie de Palo–Ullún sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.  相似文献   
7.
Metamorphism of Grenvillian age (ca. 1.2 Ga; U–Pb zircon dating) is recognized for the first time in the Western Sierras Pampeanas (Sierra de Maz). Conditions reached granulite facies (ca. 780 °C and ca. 780 MPa). Comparing geochronological and petrological characteristics with other outcrops of Mesoproterozoic basement, particularly in the northern and central Arequipa-Antofalla craton, we suggest that these regions were part of a single continental crustal block from Mesoproterozoic times, and thus autochthonous or parautochthonous to Gondwana.  相似文献   
8.
The Neoproterozoic to Early Cambrian amalgamation of SW Gondwana through the Brasiliano/Pan-African orogeny is reviewed with emphasis on the role of the Río de la Plata craton of South America in the light of new evidence from a borehole at the eastern end of the Tandilia belt (38°S). U–Pb, Hf and O isotope data on zircon indicate that this un-reworked Palaeoproterozoic craton abuts against a distinct continental terrane to the east (Mar del Plata terrane). The craton is bounded everywhere by transcurrent faults and there is no evidence to relate it to the Neoproterozoic mobile belts now seen on either side. The Punta Mogotes Formation at the bottom of the borehole contains 740–840 Ma detrital zircons that are assigned to a widespread Neoproterozoic rifting event. The data suggest that the Mar del Plata terrane rifted away from the southwestern corner of the Angola block at c. 780 Ma. Negative εHft values and δ18O > 6.5‰ suggest derivation by melting of old crust during a protracted extensional episode. Other continental terranes may have formed in a similar way in Uruguay (Nico Pérez) and southeastern Brazil, where the Schist Belt of the Dom Feliciano orogenic belt is probably a correlative of the Punta Mogotes sequence, implying that the Dom Feliciano belt must extend at least as far as 38°S. A new geodynamic scenario for West Gondwana assembly includes at least two major oblique collisional orogenies: Kaoko–Dom Feliciano (580–680 Ma) and Gariep–Saldania (480–580 Ma), the latter resulting from oblique impingement of the Rio de la Plata craton against the Kalahari craton. Assembly of this part of South-West Gondwana was accomplished before the Ordovician (to Silurian?) siliciclastic platform sediments of the Balcarce Formation in the Tandilia Belt covered the southern sector of Río de la Plata craton.  相似文献   
9.
The enigmatic Arequipa Massif of southwestern Peru is an outcrop of Andean basement that underwent Grenville-age metamorphism, and as such it is important for the better constraint of Laurentia–Amazonia ties in Rodinia reconstruction models. U–Pb SHRIMP zircon dating has yielded new evidence on the evolution of the Massif between Middle Paleoproterozoic and Early Paleozoic. The oldest rock-forming events occurred in major orogenic events between ca. 1.79 and 2.1 Ga (Orosirian to Rhyacian), involving early magmatism (1.89–2.1 Ga, presumably emplaced through partly Archaean continental crust), sedimentation of a thick sequence of terrigenous sediments, UHT metamorphism at ca. 1.87 Ga, and late felsic magmatism at ca. 1.79 Ga. The Atico sedimentary basin developed in the Late-Mesoproterozoic and detrital zircons were fed from a source area similar to the high-grade Paleoproterozoic basement, but also from an unknown source that provided Mesoproterozoic zircons of 1200–1600 Ma. The Grenville-age metamorphism was of low-P type; it both reworked the Paleoproterozoic rocks and also affected the Atico sedimentary rocks. Metamorphism was diachronous: ca. 1040 Ma in the Quilca and Camaná areas and in the San Juán Marcona domain, 940 ± 6 Ma in the Mollendo area, and between 1000 and 850 Ma in the Atico domain. These metamorphic domains are probably tectonically juxtaposed. Comparison with coeval Grenvillian processes in Laurentia and in southern Amazonia raises the possibility that Grenvillian metamorphism in the Arequipa Massif resulted from extension and not from collision. The Arequipa Massif experienced Ordovician–Silurian magmatism at ca. 465 Ma, including anorthosites formerly considered to be Grenvillian, and high-T metamorphism deep within the magmatic arc. Focused retrogression along shear zones or unconformities took place between 430 and 440 Ma.  相似文献   
10.
A‐type orthogneisses of mid Neoproterozoic age (774 ± 6 Ma, U‐Pb SHRIMP zircon age), are reported for the first time from the Grenvillian basement of the Western Sierras Pampeanas in Argentina. These anorogenic meta‐igneous rocks represent the latest event of Rodinia break‐up so far recognized in Grenvillian basement exposures across Andean South America. Moreover, they compare well with A‐type granitoids and volcanic rocks along the Appalachian margin of Laurentia (Blue Ridge), thus adding to former evidence that the Western Sierras Pampeanas Grenvillian basement was left on the conjugate rifted margin of eastern Laurentia during Rodinia break‐up and the consequent opening of the Iapetus ocean.  相似文献   
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