Time lag between metamorphism and crystallization of anatectic granites (Córdoba, Argentina)

SHRIMP and LA-ICP-MS analyses carried out on zircons from the Río de los Sauces granite revealed their metamorphic and igneous nature. The metamorphic zircons yielded an age of 537±4.8 (2σ)Ma that probably predates the onset of the anatexis during the Pampean orogeny. By contrast, the igneous zircons yielded a younger age of 529±6 (2σ)Ma and reflected its crystallization age. These data point to a short time lag of ca. 8Myr between the High Temperature (HT) metamorphic peak and the subsequent crystallization age of the granite. Concordia age of 534±3.8 (2σ)Ma, for both types of zircon populations, can be considered as the mean age of the Pampean HT metamorphism in the Sierras de Córdoba.

Structural setting of a transpressive shear zone: insights from geological mapping, quartz petrofabric and kinematic vorticity analysis in NE Sardinia (Italy)

The Posada–Asinara Line is a crustal-scale transpressive shear zone affecting the Variscan basement in northern Sardinia during Late Carboniferous time. We investigated a structural transect of the Posada–Asinara Line (Baronie) with the aid of geological mapping and structural analysis. N-verging F2 isoclinal folds with associated mylonitic foliation (S2) are the main deformation features developed during the Posada–Asinara Line activity (D2). The mineral assemblages and microstructures suggest that the Posada–Asinara Line was affected by a retrograde metamorphic path. This is also confirmed by quartz microstructures, where subgrain rotation recrystallization superimposes on grain boundary migration recrystallization. Crystallographic preferred orientation data, obtained using electron backscatter diffraction, allowed analysis of quartz slip systems and estimation of the deformation temperature, vorticity of flow and rheological parameters (flow stress and strain rate) during the Posada–Asinara Line activity. Quartz deformation temperatures of 400 ± 50 °C have been estimated along a transect perpendicular to the Posada–Asinara Line, in agreement with the syn-kinematic post-metamorphic peak mineral assemblages and the late microstructures of quartz. The D2 phase can be subdivided in two events: an early D2early phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2late phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2late deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2early deformation developed under peak temperature conditions, while the D2late event was active at shallower structural levels.

Time's arrow, time's cycle: Granulite metamorphism and geodynamics

Although the thermal evolution of the mantle before c. 3.0 Ga remains unclear, since c. 3.0 Ga secular cooling has dominated over heat production—this is time's arrow. By contrast, the thermal history of the crust, which is preserved in the record of metamorphism, is more complex. Heat to drive metamorphism is generated by radioactive decay and viscous dissipation, and is augmented by the influx of heat from the mantle. Notwithstanding that reliable data are sparse before the Neoarchean, we use a dataset of temperature (T), pressure (P) and thermobaric ratio (T/P at the metamorphic ‘peak’), and age of metamorphism (t, the timing of the metamorphic ‘peak’) for rocks from 564 localities ranging in age from the Cenozoic to the Eoarchean eras to interrogate the crustal record of metamorphism as a proxy for the heat budget of the crust through time. On the basis of T/P, metamorphic rocks are classified into three natural groups: high T/P type (T/P >775°C/GPa, mean T/P ~1105°C/GPa), including common and ultrahigh-temperature granulites, intermediate T/P type (T/P between 775 and 375°C/GPa, mean T/P ~575°C/GPa), including high-pressure granulites and medium- and high-temperature eclogites, and low T/P type (T/P <375°C/GPa, mean T/P ~255°C/GPa), including blueschists, low-temperature eclogites and ultrahigh-pressure metamorphic rocks. A monotonic increase in the P of intermediate T/P metamorphism from the Neoarchean to the Neoproterozoic reflects strengthening of the lithosphere during secular cooling of the mantle—this is also time's arrow. However, temporal variation in the P of intermediate T/P metamorphism and in the moving means of T and T/P of high T/P metamorphism, combined with the clustered age distribution, demonstrate the cyclicity of collisional orogenesis and cyclic variations in the heat budget of the crust superimposed on secular cooling since c. 3.0 Ga—this is time's cycle. A first cycle began with the widespread appearance/survival of intermediate T/P and high T/P metamorphism in the Neoarchean rock record coeval with amalgamation of dispersed blocks of lithosphere to form protocontinents. This cycle was terminated by the fragmentation of the protocontinents into cratons in the early Paleoproterozoic, which signalled the start of a new cycle. The second cycle continued with the progressive amalgamation of the cratons into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Neoproterozoic. This cycle represented a period of relative tectonic and environmental stability, and perhaps reduced subduction during at least part of the cycle. During most of the Proterozoic the moving means for both T and T/P of high T/P metamorphism exceeded the arithmetic means, reflecting insulation of the mantle beneath the quasi-integrated lithosphere of Columbia and, after a limited reorganisation, Rodinia. The third cycle began with the steep decline in thermobaric ratios of high T/P metamorphism to their lowest value, synchronous with the breakup of Rodinia and the formation of Pannotia, and the widespread appearance/preservation of low T/P metamorphism in the rock record. The thermobaric ratios for high T/P metamorphism rise to another peak associated with the Pan-African event, again reflecting insulation of the mantle. The subsequent steep decline in thermobaric ratios of high T/P metamorphism associated with the breakup of Pangea at c. 0.175 Ga may indicate the start of a fourth cycle. The limited occurrence of high and intermediate T/P metamorphism before the Neoarchean suggests either that suitable tectonic environments to generate these types of metamorphism were not widely available before then or that the rate of survival was low. We interpret the first cycle to record stabilisation of subduction and the emergence of a network of plate boundaries in a plate tectonics regime once the balance between heat production and heat loss changed in favour of secular cooling, possibly as early as c. 3.0 Ga in some areas. This is inferred to have been a globally linked system by the early Paleoproterozoic, but whether it remained continuous to the present is unclear. The second cycle was characterised by stability from the formation of Columbia to the breakup of Rodinia, generating higher than average T and T/P of high T/P metamorphism. The third cycle reflects colder collisional orogenesis and deep subduction of the continental crust, features that are characteristic of modern plate tectonics, which became possible once the average temperature of the asthenospheric mantle had declined to <100°C warmer than the present day after c. 1.0 Ga.

Ortoneises en la Franja Silos-Babega, Macizo de Santander, Colombia: evidencias de la orogenia famatiniana en los Andes del norte

The Orthogneiss unit in the Santander Massif, Northern Colombian Andes, mainly consists of quartzfeldspathic, pelitic and minor mafic orthogneisses and amphibolites. Petrographic, geochemical and geochronological studies carried out on orthogneisses from the Silos-Babega belt, indicate that they are granodioritic and granitic in composition with protolith formed by crustal melting in an active continental magmatic arc. They were syntectonically emplaced in the Silgara Schists unit. Metamorphic peak of the Orthogneiss unit reach amphibolite facies conditions in the range of 4.3-10 kbar in pressure and 540-690 °C in temperature. Zircon U-Pb LA-ICP-MS ages of 471±11 and 479±10 Ma were obtained and these ages are similar to those known for the Orthogneiss unit in the central and the eastern Santander Massif and confirm the continuity to the north of the Andean protomargin as a result of the Famatinian orogeny.

The Cora Lake shear zone, Athabasca granulite terrane, an intraplate response to far-field orogenic processes during the amalgamation of Laurentia

The Cora Lake shear zone (CLsz) is a 4–6 km wide localized high-strain zone that bisects the polydeformed Athabasca granulite terrane, northern Saskatchewan. It also coincides with the geophysical trace of the Snowbird tectonic zone. The CLsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of >20 000 km2 of high-pressure granulites. Most rocks have a strong mineral lineation plunging moderately to the southwest. The Northwestern subdomain (hangingwall) is characterized by ca. 2.6 Ga plutonic rocks that contain an early, subhorizontal gneissic layering (ca. 2.57 Ga) that was overprinted by large amplitude folds and a partitioned, but pervasive, axial planar, dextral, shear fabric at ca. 1.9 Ga. Thermobarometry suggests metamorphic conditions of ∼0.9 GPa and ∼750 °C during both of the phases of tectonism. The footwall is predominantly underlain by the ca. 3.3–3.0 Ga Chipman tonalite, layers of intercalated mafic and felsic granulite, and the widespread 1.9 Ga Chipman mafic dyke swarm. Early subhorizontal layering in the footwall was also folded at ca. 1.9 Ga and transposed into a steeply dipping, northeast-striking axial planar shear fabric that corresponds with the metamorphic peak (1.1–1.2 GPa and 800–900 °C). These distinct domains were juxtaposed across the CLsz, which contains a gneissic foliation striking 231° and dipping moderately to steeply to the northwest. Abundant sinistral–normal kinematic indicators are consistent with the distinctly lower pressures to the northwest. The shear zone is characterized by very fine grain sizes, despite its high-temperature assemblages including clinopyroxene and garnet. Thermobarometry from the CLsz displays progressive decompression of reworked footwall rocks with increasing mylonitization. In situ monazite geochronology indicates shearing at 1.89–1.87 Ga shortly after the granulite facies metamorphic peak. The anomalous sinistral kinematics of the CLsz, bracketed in time between periods of dextral shearing, can be explained by changing regional stresses during alternating convergent tectonics to the west and to the southeast of the Athabasca granulite terrane.

Major nappe-like D2 folds in the Dalradian rocks of the Beinn Udlaidh area, Central Highlands, Scotland

ABSTRACTTwo regional-scale, recumbent folds control the structure of the Beinn Udlaidh area, Tyndrum, Perthshire. They reached their maximum development during D2, following the regional metamorphic peak, and are part of a stack of larger SE-facing recumbent folds formed during the ∼470 Ma Grampian Orogeny. The rocks belong to the Neoproterozoic–Lower Ordovician Dalradian Supergroup, and preserve a sedimentary transition between the Grampian Group and the overlying Appin Group. The latter occupies the core of the S-facing, recumbent Beinn Udlaidh Syncline (D2) which, with the underlying complementary Glen Lochy Anticline, is gently folded by a regional-scale structure, the Orchy Dome. The recumbent folds postdate an early fabric (S1), which is generally obliterated by the D2 imprint, but preserved as inclusion trails in regional metamorphic garnets, that are highly oblique to, and wrapped by, S2. It is concluded that the Dalradian rocks described here from below the Iltay Boundary Slide are in structural continuity with those of the Tay Nappe above, and that the Slide represents a structurally-modified disconformity between the Leven Schist (Appin Group) and the overlying Ben Lui Schist (Argyll Group). The Orchy Dome probably influenced the spatial distribution of minor intrusions and explosion vents of the lamprophyre suite.

Wollastonite-bearing assemblages from the Dalradian at Fraserburgh, northeast Scotland and their bearing on the emplacement of garnetiferous granitoid sheets

Metasediments of the Tayvallich Subgroup of the Dalradian at Kinnairds Head, Fraserburgh are metamorphosed to sillimanite + K-feldspar grade and form part of the classic high-T low-P Buchan metamorphic terrain. Pelitic samples constrain peak-metamorphic conditions to 615±13°C and 2.2±0.2 kbar. At or close to the metamorphic peak, irregular garnetiferous aplites and autopegmatite bodies intruded the metasediments. Thin marble bands within the sequence are dominated by calcite with diopside, and equilibrated with relatively CO2-rich, internally buffered fluids. Where these are in close proximity to granitoid pegmatites, wollastonite dominates the matrix, and fractures and veins running through the rock contain concentrations of grossular and vesuvianite. With increasing distance from the pegmatite, vesuvianite and then grossular disappear, and wollastonite is only patchily developed. Such occurrences require a flushing of the marble by metasomatic (siliceous and aluminous) aqueous fluids which were derived from the de-watering of the adjacent pegmatite as it crystallized. The large quantities of dissolved silica led to pervasive wollastonite formation for several metres. The smaller quantities of Al reacted to form Ca-Al-silicates which were confined to the fractures.