280 research outputs found
High precision U-PB geochronology and implications for the tectonic evolution of the Superior Province
The underlying mechanisms of Archean tectonics and the degree to which modern plate tectonic models are applicable early in Earth's history continue to be a subject of considerable debate. A precise knowledge of the timing of geological events is of the utmost importance in studying this problem. The high precision U-Pb method has been applied in recent years to rock units in many areas of the Superior Province. Most of these data have precisions of about + or - 2-3 Ma. The resulting detailed chronologies of local igneous development and the regional age relationships furnish tight constraints on any Archean tectonic model. Superior province terrains can be classified into 3 types: (1) low grade areas dominated by meta-volcanic rocks (greenstone belts); (2) high grade, largely metaplutonic areas with abundant orthogneiss and foliated to massive I-type granitoid bodies; and (3) high grade areas with abundant metasediments, paragneiss and S-type plutons. Most of the U-Pb age determinations have been done on type 1 terrains with very few having been done in type 3 terrains. A compilation of over 120 ages indicates that the major part of igneous activity took place in the period 2760-2670 Ma, known as the Kenoran event. This event was ubiquitous throughout the Superior Province
New U-Pb zircon age constraints for the emplacement of the Reguengos de Monsaraz Massif (Ossa Morena Zone)
The Reguengos de Monsaraz massif is located in the Portuguese sector of the Ossa Morena Zone (ZOM), one of the major geotectonic units of the Iberian Variscan Belt. It intruded Lower Paleozoic metasediments, strongly affected by deformation and metamorphism during the Varis-can orogeny. Pluton emplacement was responsible for the development of a contact metamor-phism aureole in the surrounding country rocks. Based on regional constraints, the intrusion was included in the group of the late-post-tectonic granitoids.
The pluton shows an inversely zoned pattern and consists of tonalitic to granodioritic rocks with abundant mafic microgranular enclaves, occupying most of the presently exposed intrusion area and minor bodies of gabbro-diorites in the east and centre. Field, petrographical and geo-chemical data reveal that mixing / mingling between mantle- and crustally-derived magmas and fractional crystallization played a major role in the genesis and evolution of these granitoids 1.
Early attempts to date the Reguengos de Monsaraz intrusion, using Rb-Sr isotopic data for a feldspar-amphibole pair from one granodiorite sample, yielded an age of 297.5 ± 2.9 Ma. This age was interpreted as a cooling age and provided a minimum estimate for the timing of mag-matic crystallization. In order to better constrain the age and emplacement sequence of the mas-sif, five samples representing the different magmatic units (gabbro diorites, tonalites and mafic microgranular enclaves) were selected for determination of U-Pb zircon ages by isotope dilution techniques.
The zircon populations from two samples of the gabbro diorites (one from the central sector and the other from the easternmost body) yielded overlapping 206Pb/238U average ages of 337.4 1.1 Ma and 338.6 ± 0.7 Ma, respectively. Similar ages were obtained in two samples from the main tonalite-granodiorite facies (337.3 ± 2.3 Ma; 337.8 ± 0.7 Ma). Finally, the zircons from the microgranular enclave yielded a 206Pb/238U average age of 336.5 ± 0.5 Ma. A slightly younger age was obtained in two fractions of titanite from the enclave (333.6 ± 2.5 Ma), suggesting either resetting of their original magmatic ages or late stage crystallization.
The new U-Pb age data support a coeval emplacement for all the members of this suite, at ap-proximately 337-338 Ma. Such a close space-time association between mafic and felsic grani-toids is consistent with the proposed mixing model.
According to recent studies, this sector of the OMZ was affected by three Variscan deforma-tion events 2. The earlier deformation phases (D1 and D2) occurred in Late Devonian ( 380-360 Ma) and Early Carboniferous times ( 360-345 Ma), whilst the latter (D3) has a Pennsylva-nian age (305-295 Ma). Based on the available geochronological information, the emplacement of the Reguengos de Monsaraz magmas took place at a time of tectonic quiescence between D2 and D3. The relatively young Rb-Sr age recorded in the feldspar-amphibole pair may therefore reflect the effects of the last tectonothermal event, also documented by the replacement of horn-blende crystals by actinolite
Syntectonic Variscan magmatism in the Aguiar da Beira region (Iberian Massif, Portugal)
The Aguiar da Beira region (Portugal) is located in the core of the Iberian Massif, more precisely in the Central-Iberian Zone, which is dominantly composed by abundant volumes of plutonic rocks, emplaced into Late Proterozoic – Early Cambrian and Palaeozoic metasediments, mainly during or slightly after the third deformation phase of the Variscan Orogeny (D3). A considerable amount of these granitoids are syntectonic, intruded during the peak of this deformation event (D3). In particular, at the Aguiar da Beira region, there are two syntectonic granitoids that represent two distinct magmatic series: a medium- to coarse-grained porphyritic biotite granodiorite-granite (322 Ma), which belongs to the early granodiorite series, and a medium-grained muscovite-biotite granite (317 Ma) that is part of the two-mica peraluminous leucogranites suite.
The petrographical, geochemical (whole-rock and mineral compositions) and isotopical (Sr-Nd, 18O-wr and 18O-zr) study of the two intrusions revealed their remarkably different character, and allowed to conclude that they correspond to two independent magma pulses, derived from distinct sources and petrogenetic processes. The biotite granodiorite-granite is a weakly peraluminous intrusion, characterized by intermediate to felsic SiO2 contents (66 – 68%), high Ba, Sr and REE, and high Al and Mg biotite contents, typical of the calc-alkaline associations. The Sr-Nd initial ratios are homogeneous and overlap the lower crustal felsic metaigneous granulites signatures (Villaseca et al. 1999) which might suggest an origin by the anatexis of lower felsic metaigneous rocks, and is further supported by 18O-wr and 18O-zr data. However, the data, allied to the presence of microgranular enclaves present in this intrusion also raises the hypothesis of an origin by mixing of lower crustal derived magmas and mantle melts. Based on the available data these seem to be the two genetic scenarios that can best explain the geochemical signature of the biotite granodiorite-granite, not being possible to opt for any of the options. By contrast, the muscovite-biotite granite has an entirely distinct geochemical signature, typical of S-type granites: a highly evolved and strongly peraluminous character (SiO2 = 72 - 74%; CaO = 0.3-0.6%; A/CNK = 1.18 - 1.36, low Mg, Ti, Ba, Sr, REE, HFSE contents, and high Al2O3/TiO2 e Rb/Sr ratios), high (87Sr/86Sr)317 (0.71037 - 0.71459), low Ndi (-7.7 to -8.7), and high 18O (18O-wr = 11.33 ‰; 18O-zr = 9.5 ± 0.2‰). The data suggest that this magma was derived from the partial melting of metasedimentary middle crustal protoliths, which has been successfully modeled. The observed major and trace element composition variation suggests an evolution controlled by fractionation crystallization of a mineral association composed by plagioclase + biotite + apatite + zircon ± monazite ± ilmenite
Geochronology and Isotope Geochemistry of Felsic Marginal Facies of the Reguengos de Monsaraz Pluton (OMZ)
No sector sueste do maciço de Reguengos de Monsaraz (Zona de Ossa-Morena), ocorrem rochas félsicas quer em continuidade espacial com as litologias principais do plutonito quer em pequenos corpos aparentemente separados. Neste trabalho, estudou-se o granito marginal da zona de Arraieiras, bem como o tonalito da zona do ribeiro dos Gagos. Datações U-Pb (em zircão e monazite) revelam que estas litologias são, no essencial, contemporâneas entre si, bem como relativamente às fácies mais comuns do maciço de Reguengos, e que os processos de instalação magmática deverão ter decorrido há 337-338 Ma. Do ponto de vista geoquímico, o granito de Arraieiras parece ter uma relação genética forte com o resto do plutonito, através de processos de mistura de líquidos (de origens mantélica e de anatexia de metassedimentos) e diferenciação magmática, a que se terão seguido fenómenos locais de assimilação do encaixante no nível de instalação final. O paradoxo de haver rochas mais máficas do que o granito com assinaturas isotópicas mais “crustais” poderá ser explicado pela intervenção final destes processos de assimilação. Quanto ao tonalito de Gagos, as suas características geoquímicas, e em particular as razões isotópicas de Sr e Nd, requerem uma origem distinta, com contribuição de fonte infracrustal.Felsic rocks occur in the SE sector of the Reguengos de Monsaraz massif either as a border zone of the pluton or as small satellite bodies. In this work, the Arraieiras granite (from the border zone) and the Gagos tonalite (from a satellite body) were studied for geochronology and geochemistry. The obtained zircon and monazite U-Pb ages show that the two felsic lithologies and the main units of the Reguengos pluton are essentially contemporaneous to each other and that magma emplacement took place at 337-338 Ma. Geochemical features of the Arraieiras granite suggest that it has a strong genetic link with the other lithologies of the pluton, through magma mixing (from mantle and metasedimentary sources) and differentiation processes; in a late evolutionary stage, at the final emplacement level, assimilation of country rock material also took place. The paradox of some more mafic rocks displaying isotope signatures more “crustal” than those revealed by the Arraieiras granite is probably a consequence of late assimilation events. In contrast, the Gagos tonalite, according to the Sr and Nd isotope fingerprints, seems to be related to a different sequence of processes and, in particular, it requires the contribution of an infracrustal source
New U-Pb zircon ages for Early Ordovician magmatism in Central Portugal
The Mundão anatectic complex is located in the axial zone of the Iberian massif of Central Northern Portugal
(Central Iberian Zone). It consists of lenticular bodies of felsic gneisses and stromatic metatexites derived from
metasedimentary protoliths of Neoproterozoic-Lower Cambrian age, both showing evidence of incipient to
extensive partial melting during the Variscan orogeny. Although the precise age of the migmatization event is still
unknown, field and structural evidence show that the partial melting conditions were reached in the last stages
of crustal thickening (D1), continued during subsequent extensional deformation (D2) and culminated with the
emplacement of large volumes of S-type granite magmas in a transcurrent tectonic regime (D3). The upper limit
for crustal melting is provided by the Late Carboniferous age of the S-type granitoids (312.4 2.7 Ma) intruding
the Mundão anatectic complex.
Due to their transitional contacts with the stromatic metatexites, the felsic gneisses were previously interpreted as
diatexites resulting from anatexis of the same pelitic and/or metagreywacke protolith. However, new U–Pb dating
by ID-TIMS show that the zircon fractions from one sample of the leucocratic gneisses are concordant and yield
a 206Pb/238U weighted average age of 474.5 1.5 Ma. The results obtained reveal that these rocks correspond
to orthogneisses instead of sedimentary-derived diatexites and provide a good estimate for the crystallization
age of their magmatic protolith. In contrast, the monazite fractions show distinct 207Pb/235U ages of 341.8
2.1 Ma and 453.4 2.0 Ma and may therefore have lost Pb to different extent during Variscan deformation and
metamorphism.
The preservation of inherited ages of the igneous protolith in the Mundão leucocratic gneisses suggests that zircon
was mainly incorporated as restitic material, with only minor precipitation and/or recrystallization during crustal
anatexis.
The new age of the Mundão orthogneiss brackets the so-called Early Ordovician Sardic unconformity, characteristic
of the Central Iberian Zone
Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle
Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine-garnet-websterite, garnet-websterite, orthopyroxene-phlogopite-garnet-websterite, coarse-grained phlogopite-garnet-websterite, phlogopite-garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U-Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734-0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generatio
Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle
Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine–garnet-websterite, garnet-websterite, orthopyroxene–phlogopite–garnet-websterite, coarse-grained phlogopite–garnet-websterite, phlogopite–garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U–Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734–0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generation
SHRIMP ion probe zircon geochronology and Sr and Nd isotope geochemistry for southern Longwood Range and Bluff Peninsula intrusive rocks of Southland, New Zealand
Permian–Jurassic ultramafic to felsic intrusive complexes at Bluff Peninsula and in the southern Longwood Range along the Southland coast represent a series of intraoceanic magmatic arcs with ages spanning a time interval of 110 m.y. New SHRIMP U-Pb zircon data for a quartz diorite from the Flat Hill complex, Bluff Peninsula, yield an age of 259 ± 4 Ma, consistent with other geochronological and paleontological evidence confirming a Late Permian age. The new data are consistent with an age of c. 260 Ma for the intrusive rocks of the Brook Street Terrane. SHRIMP U-Pb zircon ages for the southern Longwood Range confirm that intrusions become progressively younger from east to west across the complex. A gabbro at Oraka Point (eastern end of coastal section) has an age of 245 ± 4 Ma and shows virtually no evidence of zircon inheritance. The age is significantly different from that of the Brook Street Terrane intrusives. Zircon ages from the western parts of the section are younger and more varied (203–227 Ma), indicating more complex magmatic histories. A leucogabbro dike from Pahia Point gives the youngest emplacement age of 142 Ma, which is similar to published U-Pb zircon ages for the Anglem Complex and Paterson Group on Stewart Island
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