Beatriz Levi

Multiple sources for the Coastal Batholith of central Chile (31–34°S): geochemical and Sr–Nd isotopic evidence and tectonic implications

The Coastal Batholith of central Chile between 31 and 34°S represents a locus of long-lived (c. 200 Ma) plutonic activity. It is composed of four plutonic complexes: Santo Domingo, Limari, Papudo–Quintero and Illapel. These complexes, which occur as north-trending discontinuous belts that decrease in age eastward, were emplaced during four major episodes of arc magmatism. The Santo Domingo Complex (Carboniferous) comprises hornblende–biotite bearing tonalites and granodiorites with abundant mafic enclaves, and enclave-free granites. The association of enclave-bearing granitoids and enclave-free granites resulted from different degrees of mixing of crustal- and mantle-derived magmas. The Limari Complex (Lower Jurassic) is a bimodal association of crustal leucogranites and mantle-derived gabbros. The lithology of the Papudo–Quintero Complex (Middle Jurassic) varies from hornblende–pyroxene diorite to biotite granite. The main rock types, namely tonalites and granodiorites, commonly contain mafic enclaves. The granitoids and the enclaves have remarkably similar REE patterns and are similar in Sr–Nd isotope composition, which suggest that they represent a cogenetic suite of mantle origin. The Illapel Complex (Cretaceous) consists of hornblende bearing tonalites, trondhjemites and granodiorites that resemble the rocks of TTG suites. The eNd values and initial ratios (Sri) for the mafic rocks of the Carboniferous Complex (eNd: −2.0 and −3.5; Sri: c. 0.7057) differ markedly from those of the Mesozoic mafic rocks (eNd: +1.9 to +5.4; Sri: 0.7033 to 0.7039), the Cretaceous rocks being the most depleted with regard to Sr–Nd. A change from Carboniferous lithosphere-dominated sources to Mesozoic asthenosphere-dominated sources of the magmas that started in the Early Jurassic is recognized. Slab components can be recognized in the source of the Cretaceous rocks. The crustal source of the Lower Jurassic leucogranites was more isotopically depleted and refractory than that of the Carboniferous granites. On the other hand, no crustal participation in the origin of the Middle Jurassic and Cretaceous felsic granitoids is recognized; they are differentiated from mantle-derived magmas. Progressive removal of old lithospheric mantle during the Mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source, as well as the melting and modification towards a more refractory nature of the continental crust.

Jurassic and Early Cretaceous island arc volcanism, extension, and subsidence in the Coast Range of central Chile

More than 2000 km 3 of acid and 9000 km 3 of basic volcanic rocks formed during the Jurassic and Early Cretaceous in the Coast Range of central Chile, between 32°30′S and 34°S. These rocks, which constitute the major part of an •15-km-thick pile of alternately marine and continental deposits, issued from volcanic arcs situated between a land area with Paleozoic basement in the west and a marginal sea in the east. Asthenospheric upwelling led to extension and bimodal volcanism; the volcanic products were deposited in intra-arc basins subsiding at high rates (100–300 m/m.y.). The source of the magmas became more depleted with time due to an increase in degree of partial melting, and their compositions were modified by subduction-related fluids and contamination with a progressively thinner and younger crust. The basic lavas are of high-K calc-alkaline to shoshonitic affinity, chemically resembling the lavas found in some mature island arcs in the western Pacific. The extension and subsidence resulted in a low-relief topography close to sea level, in contrast with the present-day convergent type of Andean volcanism at the same latitude where calc-alkaline intermediate lavas erupt from volcanoes at great height above a thick crust.

Zircon ages of metavolcanic and synorogenic granitic rocks from the Svärdsjö and Yxsjöberg areas, south central Sweden

Abstract U-Pb dating of zircons from the Yxsjoberg and Svardsjo leptites yields a discordia age of 1900 ± 19 Ma and for zircons from the synorogenic granite near Svardsjo, 1873+11 -10 Ma. Calculation with the systematics of Wickman (1983: Uranium-lead isotope systematics: The case of crystals with discrete cores. Lithos 16, in press) gives an age of 1925 Ma for the granite zircons.

Tertiary volcanism during extension in the Andean foothills of central Chile (33°15′–33°45′S)

This lithologic and geochemical study treats two Tertiary volcanic formations in the Andean foothills of central Chile deposited during and after an inferred culmination of crustal attenuation. The Abanico and Farellones Formations, which are described in their type localities just east of Santiago, formed from volcanic arcs in continental basins during the Oligocene and Early Miocene, respectively. Aphyric basic lavas of tholeiitic affinity, acid pyroclastic rocks, and lacustrine deposits constitute the >3100-m-thick Abanico Formation. The overlying >2100-m-thick Farellones Formation consists of calc-alkaline lavas (basalts absent) with thick pyroclastic deposits at the base. Both formations have Nd-Sr isotope signatures within the mantle array; the Abanico rocks (eNd ≈ +5.7) plot closer to N-MORB (normal mid-oceanic-ridge basalt) than the Farellones rocks (eNd = +3.9 to +5.1). The REE (rare earth element) patterns indicate greater depth to the mantle source and a smaller degree of partial melting with time. The Abanico lavas segregated within the stability field of spinel, whereas the lavas of the upper Farellones member show residual garnet in their source. Geochemical changes with time are systematic: the greatest contrast is between the middle and upper Farellones members in 1–2 m.y., e.g., for basaltic andesites, La/Yb increases from 4.3 (Abanico) to 5.6, 6.0, and 11.6 (lower, middle, and upper Farellones members). The bimodal composition of the Abanico Formation and the lower Farellones member indicates that volcanism took place during episodes of extensional conditions. Extension with subsidence is independently shown by the burial metamorphic pattern. Voluminous pyroclastic flows, structural relationships, and other evidence suggest recurrent caldera collapse. The first extensional episode ended with contraction and folding of the Abanico rocks, and the second episode resulted in uplift of the lower and middle Farellones members, followed by a more passive tectonic regime. Sequences showing many similarities with the Abanico and Farellones Formations occur along the Andean foothills of Chile. They decrease in age from Late Cretaceous– Paleocene at 23°S to Early Miocene–late Miocene at 35°S and might be explained by oblique subduction of oceanic ridge.

Metamorphic gradients in burial metamorphosed vesicular lavas: Comparison of basalt and spilite in Cretaceous basic flows from central Chile

Partial spilitization of a 9 km thick pile of flood basalts with highly vesicular flow tops gave rise to patterns of secondary mineralogy at different scales: (a) a local pattern of mineralogical variation from the almost unaltered bottom towards the altered top of each flow, and (b) an overall pattern, comparing flow tops throughout the pile, with changes in mineralogical composition within a sequence of metamorphic zones and facies. The local patterns mimic the trend of the overall pattern, but are of opposite direction and telescoped. Thus, a gradual ordering and Andepletion of the secondary “albite” and increases in the Fe*/Al ratio of epidote and pumpellyite upwards within individual flows are comparable in range to corresponding overall changes downwards throughout several kilometres. The mineralogical changes within the flows diminish in range towards the more altered deeper part of the pile.The local and overall patterns cannot be interpreted in terms of grade. They represent trends from metastable towards stable equilibrium, this latter only approached in the flow tops of the lower part of the pile. The patterns of secondary mineralogy were formed by an interplay of metamorphic gradients at different scales at any given time, and as burial proceeded. The overall pattern was caused by depth-controlled gradients: increasing Pfluid, temperature and temperature-induced increase of reaction rates, and decreasing fO2 (downwards in the pile). The local patterns resulted from permeability-controlled gradients: increasing reaction rates, fO2 and contrast in chemical activity between different domains, and decreasing Pfluid (upwards in each flow). The mineralogical observations reported in this paper fall into line if the overall temperature-induced increase of reaction rates and the local permeability-controlled rate factors played the leading role during burial metamorphism of the pile.

Unconformities as mineralogical breaks in the burial metamorphism of the Andes

Stratigraphical-structural units separated by regional unconformities in the Andes of Peru and Chile, display a pattern of low grade burial metamorphism. Each stratigraphical-structural unit shows a particular facies series covering part or all the range between the zeolite and the greenschist facies. These facies series were episodically generated as part of the geological evolution of each unit prior to its own folding. Mineralogical breaks are found to coincide with the regional unconformities and often cases of higher grade assemblages on top of lower grade ones occur. This pattern may be explained by a process of “ sealing” of each unit after its particular metamorphic episode took place. Porosity and permeability conditioning Pf, as demonstrated for individual lava flows, are the significant controlling factors in the production of the metamorphic assemblages.

Geochemical trends in Mesozoic-Tertiary volcanic rocks from the Andes in central Chile, and tectonic implications

Abstract A study of approximately 300 samples of basic to intermediate lavas of Mesozoic and Tertiary age from five profiles across the Andes of central Chile (25°30′–35°S) shows that the ascribed “normal” geochemical trends transverse to this active continental margin and, with time (up to the Miocene), are oversimplified or reversed. The Jurassic-Paleogene lavas show no clear trend towards more evolved compositions in the east (away from the paleotrench), nor with time. There are even indications that the crust was thicker towards the south during part of the Mesozoic, contrary to the present situation. The tectonic evolution during the Jurassic-Early Cretaceous is characterized by episodic changes in the subduction regime, expressed among other things by a recurring non-organic shoshonitic fissure volcanism. The Late Cretaceous history constitutes a transition to the Cenozoic development with calcalkaline volcanic rocks erupting from central volcanoes in a caldera-graben setting.

Rb-Sr dating of alteration events in granitoids

Veins and other effects of low-grade metamorphism are often found in igneous rocks. A method for dating such veins is proposed. It is based on the presence of Ca-bearing, Rb-free minerals in the veins and their aureoles. The method is applicable if five conditions are satisfied. The effects of low-grade metamorphism on Rb-Sr dating of the emplacement (extrusion) age are also studied.Veins and their aureoles in granitoid rocks from Central Sweden are used as a working example. Two groups of vein ages were found, 1600-1500 Ma and 1250-1100 Ma, which coincide with the two known periods of tensional movements in the region.The ratio 87Sr/86Sr in the aureoles and their neighbourhood at the time of vein formation (Sr(a)) shows several different patterns. One with constant Sr(a), independent of the distance to the vein, indicates complete homogenization at the time of alteration. Another shows an increase in Sr(a) close to the vein, indicating that it was rejuvenated. Furthermore, the Sr ratio might determine whether an alteration effect could be of deuteric origin or not: deuteric effects should be characterized by a Sr(a) value equal to the initial Sr-isotope ratio.

The Götemar granite — isotopic and geochemical evidence for a complex history of an anorogenic granite

Abstract The Gotemar massif, located in south-eastern Sweden, consists mainly of a coarse-grained homogeneous alkali feldspar granite with subordinate fine-grained varieties. A multi-radiometric dating approach gives a variety of discordant data depending on method and material. The coarse-grained Gotemar granite is believed to have intruded and crystallized between 1400 and 1350 Ma ago. The isotopic data, together with Ti, Zr, Y and Nb ratios, strongly resemble those of the Jungfrun granite which represents another anorogenic intrusion in the area. This suggests a common origin for these two granites. Rb-Sr dating of the fine-grained aplitic varieties of the Gotemar granite gives an age of 1255 Ma. It is not clear whether these represent late differentiates, or if their age is lowered by late-stage hydrothermal activity.

Pumpellyite-bearing Precambrian rocks and post-Svecokarelian regional metamorphism in central Sweden

Abstract Pumpellyite-bearing mineral assemblages, formed during one or more episodes of non-deformational regional metamorphism, have been found in post-Svecokarelian rocks of central Sweden. Two different sub-greenschist facies have been established: the prehnite-pumpellyite and the pumpellyite-actinolite facies, in Jotnian and sub-Jotnian rocks respectively. It is here suggested that the common overprinting of post-deformational ‘retrograde’ minerals on amphibolite facies assemblages in Svecokarelian rocks of central Sweden could, at least in part, be attributed to the metamorphic episode(s) mentioned above. Furthermore, ‘deuteric’ minerals in granitoids of the same area could have the same origin.