Mafic Composition Research Articles

Thickness, composition, and evolution of the Indian Precambrian crust inferred from broadband seismological measurements

We present seismological constraints to model the evolution of Indian Precambrian crust through receiver function analysis of teleseismic waveforms, recorded on 22 broadband seismographs along a N–S corridor, from southern granulite terrain to Delhi, on the Aravalli craton. Archean terrains (Eastern Dharwar, Bastar, Bundelkhand and Aravalli cratons) are underlain by crust with felsic-intermediate composition ( V p/ V s ∼ 1.65–1.75) in contrast to mafic composition ( V p/ V s > 1.78) of crust beneath Vindhyan basin, Godavari basin, and the Narmada-Son Lineament (NSL) of Proterozoic age. The Moho depth varies between 33 and 43 km with no definite relationship with age of the crust. Duplex Moho with highly mafic underplated lower crust is characteristic feature of the NSL, signifying its paleo-rift character. Present day felsic character of the Indian Archean crust could be a consequence of post-formation modification of the initial mafic lower crust through the process of lithospheric delamination. This major tectonic event also created layered lithosphere beneath parts of India imprinted as positive velocity contrast at a depth of ∼80 km, usually referred to as Hales discontinuity. The upper part of the lithosphere has shear velocity ( V s) ∼4.52 km/s and the lower one ∼4.77 km/s. We speculate that such a layered lithosphere could be due to a low-density, depleted spinel peridotite (with Archean affinity) lying over higher density fertile garnet peridotite of Proterozoic age.

Mineralogical structure of the subsurface of Syrtis Major from OMEGA observations of lobate ejecta blankets

This paper focuses on the spectral characteristics of lobate ejecta across Syrtis Major volcano, a dark region of Mars presenting a mafic composition as revealed from the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument. Two spectrally distinct crater types are identified. Type I is enriched in high‐calcium pyroxene (HCP) relative to the volcanic rocks. Type II is similar to the lava flow mineralogy driven by HCP, compared to the Noachian crust dominated by low‐calcium pyroxene (LCP). Type I craters are systematically younger than type II. The type II mineralogy has been likely affected by a long‐term weathering in a cold environment for the last 2 Ga. The axisymmetric spectral signatures of type I ejecta appear dominated by rock‐forming minerals rather than by soils and reflect the composition of excavated materials offering a window for exploring the subsurface. A progressive change in the slope of the spectra around 1.5–2 μm is observed across the ejecta layers, pointing at a change in the HCP/LCP ratio. The deconvolution of the spectra by the Modified Gaussian Model unravels a maximum of HCP/LCP band strength ratio located between 1.2 and 3 crater radii for all type I craters. Using Z‐modeling, this observation translates into a maximum in the HCP/LCP abundances at a few hundreds of meters depth and suggests a homogeneous subsurface structure of the volcanic edifice. Below this horizon the HCP/LCP decrease may reveal a more ancient lava composition or the signature of the underlying Noachian crust.

Petrology and Tectonics of the Derewo Metamorphic Belt, West New Guinea

The Derewo Metamorphic Belt (also known as the Ruffaer Metamorphic Belt) is a >500 km long, narrow (~10-30 km wide), roughly E-W-trending terrane composed mostly of slate and phyllite, on the northern flank of the Central Range of Papua, Indonesia (west New Guinea). This investigation, the first systematic petrologic study of the belt, is based on petrographic and mineral chemical analysis of 133 samples collected across the 1:250,000 scale Hitalipa Geologic Map Sheet. The southern edge of the Derewo Metamorphics (DM) is a structural boundary, the Derewo fault, in the western half of the map and gradational with unmetamorphosed passive margin strata in the eastern half. The northern boundary of the DM is a fault contact with the Irian Ophiolite Belt. The metamorphic protoliths are Jurassic-Cretaceous Australian passive-margin strata (Kembelangan Group). Most of the rock in the DM is of pelitic bulk composition. The common mineral assemblage is quartz + white mica + chlorite + pyrite + graphite, with many samples containing albite, mixed layer muscovite/paragonite, and/or calcite. Spessartine garnet and iron-rich chloritoid are present but rare. Volumetrically minor protoliths were siltstones or sandstones and rare probable mafic ash beds. Metamorphosed silt- and sandstones contain quartz + albite + potassium feldspar + white mica ± biotite ± calcite. Mafic compositions contain actinolite + epidote + chlorite + albite + white mica + calcite + quartz + titanite. Thermobarometry based on mineral assemblages and compositions indicates the DM in the Hitalipa area experienced peak metamorphic conditions of 250-350°C at 5-8 kbar (burial depths of 15 to 25 km). The DM formed as continental rise and slope sediments that entered a north-dipping subduction zone since 30 Ma. Major uplift from collisional orogenesis began at ~8 Ma when positively buoyant Australian continent-capped lithosphere began to bend downward to subduct. The present-day high topography of the Central Range was established by ~4 Ma when delamination of the subducting plate was complete, and collisional movements changed into a left-lateral transform fault system. Tens of km of strike-slip displacement occurred within the core of the Central Range, offsetting parts of the metamorphic belt along the Derewo and related faults.

Provenance of Precambrian Fe‐ and Al‐rich Metapelites in the Yenisey Ridge and Kuznetsk Alatau, Siberia: Geochemical Signatures

Abstract: Major, trace and rare earth element contents of Fe‐ and Al‐rich metapelites from the Korda (Yenisey Ridge) and Amar (Kuznetsk Alatau) formations were determined to examine the nature, origin and evolution of their protoliths. Results indicate that these rocks are the redeposited and metamorphosed products of Precambrian kaolinitic weathering crusts, while the geochemical distinctions between the studied metapelites are determined by different weathering conditions in the source area and tectonic settings. The protolith of the Korda Formation metapelites was produced by erosion products of the post‐Archean granitoid rocks, which accumulated under humid climate conditions in shallow‐water basins along the continental margin. The geochemical characteristics of the deeper primary deposits of the Amar Formation suggest that volcanogenic material of mafic composition derived from an island‐arc environment had a major role in supplying the erosion zone. These results agree with lithofacies data and with the geodynamic reconstruction of the evolution of the Yenisey Ridge and Kuznetsk Alatau during the Mesoproterozoic and Neoproterozoic, respectively. It was shown that REEs had limited mobility during contact metamorphism. The coherent mobility of REEs during collisional metamorphism may be attributed both to mineral reactions responsible for modal changes and to local chemical heterogeneity inherited from the initial protolith.

Crustal structure of the south-central Andes Cordillera and backarc region from regional waveform modelling

SUMMARY We investigate the crustal structure in the Andes Cordillera and its backarc region using regional broadband waveforms from crustal earthquakes. We consider seismic waveforms recorded at regional distances by the CHile-ARgentina Geophysical Experiment (CHARGE) during 2000‐2002 and utilize previous seismic moment tensor inversion results. For each single station-earthquake pair, we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation Cordillera and the accreted terranes in the backarc region. Our investigation indicates that synthetic seismograms for our earthquake-station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters. We performed a grid search around crustal thickness, P-wave seismic velocity (Vp) and P -t oS-wave seismic velocity ratio (Vp/Vs), fixing mantle parameters. We evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components. We identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s. Our results indicate along strike variations in the crustal structure for the north‐south high Cordillera with higher P-wave velocity and thickness in the northern segment (north of 33 ◦ S), and persistently high Vp/Vs ratio (>1.85) in both segments. This is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region. In contrast, the results for the current volcanic arc (south of 33 ◦ S) agree with a warmer crust consistent with partial melt related to Quaternary volcanism presumably of an intermediate to mafic composition. A distinctive feature in the backarc region is the marked contrast between the seismic properties of the Cuyania and Pampia terranes that correlates with their heterogeneous crustal composition. The Cuyania terrane, composed of mafic-ultramafic rocks, exhibits high Vp, high Vp/Vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern Sierras Pampeanas associated with low Vp and low Vp/Vs. These differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper ∼30 km in this active compressional region. In particular, the seismic properties of the Cuyania terrane, which shows evidence for a high Vp and high Vp/Vs crust, may be reflecting the complex tectonic evolution history of this terrane including accretion-rifting and re-accretion processes since the Palaeozoic that promote a high level of crustal seismicity in the upper ∼30 km, enhanced by the flat slab subduction in the segment between 31 ◦ S and 32 ◦ S. Another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the Cuyania terrane between 30 ◦ S and 34 ◦ S.

Explanatory notes to the Geological Map of Greenland, 1:100 000, Ussuit 67 V.2 Nord

The Ussuit map area is situated around the inner Nassuttooq (Nordre Strømfjord) in central West Greenland, in the core of the Palaeoproterozoic Nagssugtoqidian orogen. The orogen largely consists of reworked Archaean gneisses, as well as Palaeoproterozoic ortho- and paragneisses in its central part. Easy access through the branched fjord system and good exposures along its coastlines, combined with less intense reworking compared to neighbouring areas to the west and south, have made the Ussuit map area the most intensely studied and best known part of the orogen. The most recent research and mapping projects were organised by the Danish Lithosphere Centre (1994–1999) and the Geological Survey of Denmark and Greenland (2000–2001).
 The predominant rocks are late Archaean tonalitic and granodioritic orthogneisses, intruded by postkinematic granites. Archaean supracrustal rocks are predominantly of mafic composition, but only few have been recognised. Palaeoproterozoic rocks occur as tectonic sheets (the Ussuit unit) which are always in tectonic contact with the Archaean orthogneisses. The most abundant Palaeoproterozoic rock types are biotite schist and biotite-bearing paragneiss, besides orthogneiss of the Arfersiorfik intrusive suite. The latter rocks are mainly deformed quartz diorites intrusive into the metasedimentary rocks and interpreted as remnants of a magmatic arc above a subduction zone. The Ussuit unit also includes amphibolite, marble and calc-silicate rocks, and lenses of ultrabasic rocks. Small bodies of younger syn- and post-tectonic granites occur throughout the map area. The interleaved Palaeoproterozoic and Archaean rocks form a major anticlinal fold structure between two crustal-scale shear zones, the Nordre Strømfjord shear zone in the north and the Nordre Isortoq steep belt in the south. These shear zones formed during the latest ductiledeformation event (D4), following ductile thrusting (D1) and kilometre-scale folding (D2 and D3). The deformation and high grade metamorphism are the result of collision of two Archaean blocks at c. 1850 Ma, with a presumed strongly deformed suture rooted in the southern Ussuit area.
 No economically feasible mineral occurrences have been discovered to date within the Ussuit map area. Minor sulphide mineralisation related to hydrothermal activity occurs in faults and shear zones, and minor stratabound iron formations have been observed. The most promising industrial mineral deposits are minor diopside occurrences and potential dimension stone in migmatised orthogneiss.

Explanatory notes to the Geological Map of Greenland, 1:100 000, Ussuit 67 V.2 Nord

The Ussuit map area is situated around the inner Nassuttooq (Nordre Strømfjord) in central West Greenland, in the core of the Palaeoproterozoic Nagssugtoqidian orogen. The orogen largely consists of reworked Archaean gneisses, as well as Palaeoproterozoic ortho- and paragneisses in its central part. Easy access through the branched fjord system and good exposures along its coastlines, combined with less intense reworking compared to neighbouring areas to the west and south, have made the Ussuit map area the most intensely studied and best known part of the orogen. The most recent research and mapping projects were organised by the Danish Lithosphere Centre (1994–1999) and the Geological Survey of Denmark and Greenland (2000–2001).
 The predominant rocks are late Archaean tonalitic and granodioritic orthogneisses, intruded by postkinematic granites. Archaean supracrustal rocks are predominantly of mafic composition, but only few have been recognised. Palaeoproterozoic rocks occur as tectonic sheets (the Ussuit unit) which are always in tectonic contact with the Archaean orthogneisses. The most abundant Palaeoproterozoic rock types are biotite schist and biotite-bearing paragneiss, besides orthogneiss of the Arfersiorfik intrusive suite. The latter rocks are mainly deformed quartz diorites intrusive into the metasedimentary rocks and interpreted as remnants of a magmatic arc above a subduction zone. The Ussuit unit also includes amphibolite, marble and calc-silicate rocks, and lenses of ultrabasic rocks. Small bodies of younger syn- and post-tectonic granites occur throughout the map area. The interleaved Palaeoproterozoic and Archaean rocks form a major anticlinal fold structure between two crustal-scale shear zones, the Nordre Strømfjord shear zone in the north and the Nordre Isortoq steep belt in the south. These shear zones formed during the latest ductiledeformation event (D4), following ductile thrusting (D1) and kilometre-scale folding (D2 and D3). The deformation and high grade metamorphism are the result of collision of two Archaean blocks at c. 1850 Ma, with a presumed strongly deformed suture rooted in the southern Ussuit area.
 No economically feasible mineral occurrences have been discovered to date within the Ussuit map area. Minor sulphide mineralisation related to hydrothermal activity occurs in faults and shear zones, and minor stratabound iron formations have been observed. The most promising industrial mineral deposits are minor diopside occurrences and potential dimension stone in migmatised orthogneiss.

Shoshonitic magmatism and the formation of the Northparkes porphyry Cu – Au deposits, New South Wales

Four economic porphyry Cu – Au deposits occur at Northparkes, New South Wales: Endeavour 22 (E22), E26, E27 and E48. Mineralisation is centred on thin, pipe-like Late Ordovician to Early Silurian quartz monzonite porphyry complexes. Nine intrusive phases have been recognised, with a common sequence of emplacement recognised in all deposits. Pre-mineralisation intrusions include coarse-grained, equigranular monzodiorite and equigranular to weakly porphyritic biotite – quartz monzonite (U – Pb SHRIMP age 444.2 ± 4.7 Ma). Three variably felsic quartz monzonite porphyry phases comprise the mineralised intrusive complexes: (i) volumetrically minor early and late mineralisation biotite-phyric quartz monzonite porphyry dykes; (ii) abundant synmineralisation K-feldspar-phyric quartz monzonite porphyry intrusions; and (iii) less abundant syn- to late mineralisation augite – biotite K-feldspar-phyric quartz monzonite porphyry intrusions. Post-mineralisation basaltic trachyandesite dykes and augite-phyric monzonite porphyry dykes are also present (U – Pb SHRIMP age 436.7 ± 3.3 Ma), as are younger mafic dykes. The regional volcanic and intrusive rocks define systematic geochemical trends consistent with high-temperature magmatic fractionation of basaltic trachyandesite through trachyte. However, the trace-element compositions and REE patterns of the mineralising intrusions cannot be explained by crystal fractionation alone because there is a return to more mafic compositions in the waning stages of intrusive activity. The intrusive complexes are interpreted to have formed due to the emplacement of mafic alkaline melts into the base of a crystallising, zoned, monzodiorite to monzonite magma chamber. Shallow crustal fault ruptures above the magma chamber probably caused instantaneous depressurisation and simultaneous egress of quartz monzonite porphyry and exsolved aqueous fluid into dilatant zones. Localised fracturing and additional volatile exsolution from the quartz monzonite melt is thought to have led to the formation of the quartz monzonite porphyry complexes and associated Cu – Au-bearing stockwork veins and related orthoclase alteration. Volatile-rich aqueous fluid partitioned LREE preferentially to MREE and HREE resulting in the development of distinctive U-shaped REE patterns of the ore-related intrusions.

Recycling of deeply subducted continental crust in the Dabie Mountains, central China

Post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone show evidence for derivation from a mantle source that was overprinted by partial melts derived from subducted crust of the South China Block. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and depletion of high field strength elements (HFSE, e.g., Nb, Ti). The Zhujiapu intrusion developed in the North Dabie zone has moderately enriched 87Sr/ 86Sr i (0.70605–0.70827), negative ε Nd( T) values (− 2.3 to − 18.8), and unradiogenic Pb isotopes. Such typical “continental” geochemical features do not result from crustal contamination or magma mixing, but reflect the properties of the mantle source. Because the trace element and Sr–Nd isotopic compositions of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone are similar to those of the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block, it is unclear whether the “continental” features are due to the contribution of lithospheric mantle and lower crust from the North China Block or deeply subducted materials from the South China Block. However, for a given 206Pb/ 204Pb i or Δ7/4, the 208Pb/ 204Pb i or Δ8/4 (159–180) of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone is close to the Mesozoic granitoids, eclogites, and gray gneisses from the Dabie–Sulu orogen, but significantly higher than the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block (Δ8/4 < 160). This indicates that the subducted slabs of the South China Block experienced a higher time-integrated Th/U but similar U/Pb as the deep lithosphere of the North China Block. We therefore propose that the mantle source of the post-collisional mafic–ultramafic intrusive rocks from the Dabie–Sulu orogen result from the depleted upper mantle metasomatised by melts from recycled deeply subducted crust. Due to the continent–continent collision between the South China Block and North China Block in the late Triassic, the lithosphere of the Dabie orogen was over-thickened resulting in eclogitic metamorphism in the mafic lower crust, producing a gravitational instability. The subducted eclogitic lower crust foundered and was recycled into the upper mantle beneath the orogen, resulting in mountain root removal, less mafic composition of unusually evolved continental crust, and upper mantle heterogeneity.

Characteristics of a kilometer-scale high strain zone in the lower continental crust: Mt. Hay block, central Australia

A granulite facies high strain zone was studied on Capricorn Ridge, part of the Mt. Hay block granulites in the Arunta inlier, central Australia. Deformation occurred in these primarily mafic composition granulites at lower crustal conditions (∼8 kb, ∼800 °C) at 1740–1690 Ma. Pervasive and consistently oriented foliation (striking SSW and dipping steeply) and lineations (pitching steeply eastward) occur within the shear zone. The high strain zone is recognized by the presence of SL-tectonites, parallelism of grain shape fabrics and lithologic boundaries, transposition of lithologic boundaries into parallelism with the foliation, and the local occurrence of sheath-like folds. Field-based fabric mapping uses compositional foliation intensity as a proxy for finite strain to document strain variations within the Capricorn Ridge high strain zone. There is no observed microstructural grain size reduction that corresponds to these strain variations. Fabric intensity increases from the centers of the major lithologic units towards the contacts. 100 m-scale strain localization within Capricorn ridge occurs adjacent to major lithologic contacts, indicating competency contrast during lower crustal deformation.

Crustal structure and evolution of the Mariana intra-oceanic island arc

A new high-resolution velocity model of the Mariana arc-backarc system obtained from active-source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions. The characteristics of the oceanic-island-arc crust are a middle crust with velocity of ∼6 km/s, laterally heterogeneous lower crust with velocities of ∼7 km/s, and unusually low mantle velocities. Petrologic modeling suggests that the volume of the lower crust, composed of restites and olivine cumulates after the extraction of the middle crust, should be significantly larger than is observed, suggesting that a part of the lower crust, especially the cumulates, is seismically a part of the mantle.

Peculiarities in the behavior of trace and rare earth elements during granitization and migmatization of the Elgakan complex (East Siberia, Nyukzha River)

The behavior of major elements in the process of Neoarchean amphibolite-facies metamorphism was considered previously for occasionally pyroxene-bearing biotite‐amphibole‐plagioclase schists and amphibolites, which occur in the middle and lower reaches of the Nyukzha River and are identified as the Elgakan Unit [1]. Ultrametamorphism took place in several stages. At the first stage, plagiogranite gneisses (Lc 1 ) and rocks of an intermediate composition (sch*) were sequentially developed from the volume replacement of schists (sch). At the second stage, the stromatic, network, and coarse-banded migmatite leucosomes (Lc 2 , Lc 3 , and Lc 4 , respectively) developed progressively in schists and plagiogranite gneisses [1, 2]. The two stages were separated by emplacement of intermediate and silicic dikes and minor intrusions. This paper is focused on behavior of trace and rare earth elements in these processes. Without dwelling on the major element geochemistry of crystalline schists and amphibolites (27 analyses), let us note that their compositions correspond to basalt‐trachybasalt and basaltic andesite‐basaltic trachyandesite in the TAS diagram: SiO 2 50.10 ± 3.50 and 55.40 ± 1.85 wt %, respectively; (Na 2 O + K 2 O) 5.22 ± 1.86 and 5.48 ± 1.27 wt %, respectively. Correlation links are typical of igneous rocks of this composition: Si displays negative correlation with Fe 3+ , Fe 2+ , Mn, Mg, and Ca; positive correlation with Na; and the absence of significant correlation with K, Ti, and Al. The absence of significant positive correlation with Ca, Na, and Al testifies to the insignificant effect of plagioclase fractionation. The Lc 1 group (26 samples) includes plagiogranite and granite gneisses with biotite (occasionally, amphibole). In terms of composition, this group varies from tonalite to trondhjemite and granite with predominance of rocks containing 68‐73 wt % SiO 2 . In terms of proportions of normative Ab, An, and Or, the distribution of samples is as follows: trondhjemite 39%, granite 23%, and tonalite and granodiorite 19% each. The sum total of alkali metals shows a positive correlation with K and a negative correlation with Na, resulting in a negative correlation between K and Na (‐0.60). In terms of ASI values, most samples fall within a range of 0.9‐1.2 (maximum 1.0‐1.1). Four samples (out of 26) are characterized by ASI = 1.2‐1.4. The Fe index ( F ) varies from 50 to 70%, and the degree of Fe oxidation ( f 0 ) is 10‐40% (maximum 20‐30%). Si has negative correlation with most of the major elements but no correlation with Na and K. Leucosomes Lc 2 (20 samples) always contain >68 wt % SiO 2 . In comparison with Lc 1 , they are characterized by the prevalence of trondhjemite (50%), a higher content of granite (35%), and a lower content of tonalite (15%). The correlation between K and Na is negative (‐0.58). Samples with ASI = 1.0‐1.1 and 1.2‐ 1.3 are most abundant. In terms of F and f 0 , these rocks are close to Lc 1 but marked by more mafic compositions and higher percentages of rocks with elevated contents of alkali metals. Si is negatively correlated with Al, Fe 2+ , and Mg. Rocks of group Lc 3 (8 samples) are similar to Lc 1 in composition. They differ from Lc 2 in their higher Mg and lower Si contents. Group Lc 4 (14 samples) is close in many parameters to Lc 2 . However, the percentage of granite is higher (64%), while tonalite (7%) and trondhjemite (29%) are subordinate. K and Na are not correlated. As in the preceding group, Si lacks any significant correlation with all major elements except Al. The samples are uniformly distributed in three ASI intervals (1.0‐1.1, 1.1‐1.2, and 1.2‐1.3). The maximums of F (60‐70%) and f 0 (20‐30%) are distinct. The compositional variation of basic rocks during the granitization and evolution of leucosomes may be traced by comparing the average major element contents in the rock series from the basic protolith to the granitized basic rocks, plagiogranite gneisses, and leu

Evidence of Early to Middle Ordovician arc volcanism in the Cordillera Oriental and Altiplano of southern Peru, Ollantaytambo Formation and Umachiri beds

Volcanic rocks of intermediate to predominantly mafic composition are exposed in the Cordillera Oriental and Altiplano of southern Peru in two localities: (1) the Ollantaytambo Formation, which crops out near Cusco, and (2) the Umachiri beds in the Departamento de Puno. The most notable unit in the Ollantaytambo Formation is an approximately 100 m thick succession of lapilli tuffs that originated by hydroclastic fragmentation. These volcanic rocks are of calc-alkaline basaltic to andesitic composition. The tuffs are overlain by fine-grained volcaniclastic sandstones and shales, which grade into quartz-rich turbidites. The Umachiri beds consist of monomict lapilli tuffs with varying lapilli abundance and size, formed by hydroclastic fragmentation, and volcaniclastic sandstones. The rocks have a uniform basaltic andesite composition and tholeiitic affinity. Chondrite-normalized element patterns of both volcanigenic units show a moderate enrichment of LREE and a weak negative Eu anomaly. Pronounced negative Nb and Ta anomalies indicate magma evolution in an arc environment. In contrast, the overlying epiclastic sandstones and shales in the Ollantaytambo Formation reflect upper crustal sources. Our data indicate that the deposition of the lapilli tuffs of the Ollantaytambo Formation and Umachiri beds likely was connected to the westward-lying Ordovician magmatic arc on the Arequipa Massif. If the Ordovician active margin in southern Peru had the same east-dipping subduction polarity as the active Gondwana margin in Argentina and Chile, the sedimentary basin in which the Ollantaytambo Formation and Umachiri beds were deposited was in a backarc position.

Unusual mantle Poisson's ratio, subduction, and crustal structure in central Alaska

Subduction zones alter the upper plate in a variety of ways, including metasomatism via slab‐derived fluids and accretion of exotic terranes. These processes should produce distinctive seismic signatures, as seismic velocities are sensitive to compositional variations. A new method is developed to estimates depths to interfaces and Poisson's ratio (Vp/Vs ratio) for dipping layered structures, by stacking teleseismic receiver functions. It also estimates crustal Vp/Vs and thickness. Using this method, we analyze a broadband data set from the central Alaska subduction zone, to seek evidence for mantle wedge composition. Crustal thickness varies from 27 to 45 km, and crustal Vp/Vs indicates an intermediate to mafic composition, expected for these accreted island arc terranes. Inversions for mantle structure confirm the presence of a 15–20 km thick low‐velocity zone atop of the downgoing plate to 130 km depth, perhaps subducted crust of an exotic terrane. Vp/Vs in the mantle wedge allows for 15 ± 15% serpentinization where the slab is &lt;80 km deep and the wedge is cold. Where the slab is deeper, Vp/Vs for the mantle wedge is unusually low, &lt;1.7. This value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz, provided that past physical properties measurements are not grossly in error. These results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle, perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle.

Geochemical stratigraphy and magmatic evolution at Arenal Volcano, Costa Rica

Arenal has been active for at least the past 7000 years. Prior to 3000 years B.P. (before present), Arenal eruptive products consisted of lavas and tephras both with a mafic (basaltic andesitic) composition. At approximately 3000 years B.P. Arenal began producing two discrete tephra compositions, a mafic (basaltic) tephra and a silicic (andesitic to dacitic) tephra as well as the basaltic andesitic lavas whose composition falls into the gap between the two tephra compositions. The amount of phenocrysts in both the mafic tephras and, to a lesser extent, the lavas, has increased steadily over time, demonstrating a gradual increase in the amount of crystal–liquid fractionation. At approximately 3000 years B.P. the fractionation reached a threshold causing the production of silicic tephras in conjunction with the mafic tephras. Modal analyses show that while the mafic tephras become more crystalline over time, the silicic tephras have become glassier. These younger mafic tephras are mineral-rich residues, while the silicic tephras are the incompatible element rich melt. There is also an increase in the crystal–liquid fractionation within the magma forming the lavas and tephras. The concentration of incompatible elements in the lavas has increased over time, while it has decreased in the tephras. In addition to the crystal–liquid redistribution in the magmas at Arenal, there have also been changes in Arenal's source region. The amount of flux from the subducting slab and consequently the degree of melting increase up to approximately 3000 years B.P., and then begin decreasing.

Spatial, temporal and geochemical characteristics of Silurian collision-zone magmatism, Newfoundland Appalachians: An example of a rapidly evolving magmatic system related to slab break-off

Silurian plutonic suites in the Newfoundland Appalachians include abundant gabbro, monzogabbro and granite to granodiorite and lesser quartz diorite and tonalite. Most are medium- to high-K, but included are some low-K and shoshonitic mafic compositions. Felsic rocks are of both alkaline (A-type or within-plate granite (WPG)) and calc-alkaline volcanic arc granite (VAG) affinity. Mafic rocks include both arc-like (Nb/Th < 3) calc-alkaline and non-arc-like (Nb/Th > 3) transitional calc-alkaline basalt to continental tholeiitic affinity compositions. ε Nd( T) values range from − 9.6 to + 5.4 and δ 18O (VSMOW) values range from + 3.1 to + 13.2‰. A rapid progression from exclusively arc-type to non-arc-like mafic and then contemporaneous WPG plus VAG magmatism has been documented using precise U–Pb zircon dating. Earlier arc-like plutonism indicates subduction, while asthenosphere-derived mafic magmas support slab break-off, due to subduction of a young, warm back-arc basin. Contemporaneous mafic magmas with arc and non-arc geochemical signatures may reflect tapping of asthenospheric and subcontinental lithospheric mantle (SCLM) sources and/or contamination of asthenosphere-derived magmas by SCLM or crust. The brevity (< 5 Ma) of the mafic magmatic pulse agrees with the transient nature of magmatism associated with slab break-off. The subsequent ca. 1 to 2 m.y. period of voluminous WPG and VAG plutonism likely reflects mafic magma-driven partial melting of both SCLM and crustal sources, respectively. Continuation of VAG-like magmatism for an additional 2 to 5 m.y. may reflect lower solidus temperatures of crustal materials, enabling anatexis to continue after mantle melting ceased. East to west spatial variation of ε Nd and (La/Yb) CN in Silurian plutons suggests a transition from shallow melting of juvenile sources proximal to the collision zone to deeper melting of old source materials in the garnet-stability field further inboard. Previous work has demonstrated that geochemical discriminaton of post-collisional granitoid magmatism (PCGM) is difficult in the absence of other constraints. Our example should contribute to the understanding and identification of PCGM if it can be employed as a ‘fingerprint’ for slab break-off-related PCGM within the Paleozoic geological record.

Mafic pyroclastic flows at Tyrrhena Patera, Mars: Constraints from observations and models

Tyrrhena Patera (22°S, 104°E), Mars, is a low-relief (1800 m maximum relief), central-vent volcano whose flanks are dissected with broad (∼5 km wide), flat-floored channels. The easily eroded nature of the shield materials, combined with extremely low flank slopes (≤ 2.0°), suggest that Tyrrhena Patera is composed of pyroclastic materials. These shield deposits are observed to crop out ≥ 600 km from the volcano's summit with measured thicknesses of 100–150 m at these distances. Such dimensions strongly suggest emplacement via pyroclastic flows. Currently available data indicate that these deposits have a mafic composition. Application of models derived for terrestrial pyroclastic flow emplacement reveals that, aside from environmental parameters (atmospheric density, gravity), if all else is held equal, a pyroclastic flow should travel a factor of 3 to 4 times farther on Mars than on Earth. The observed Tyrrhena Patera deposits extend even farther, suggesting that some emplacement mechanism may be enhancing pyroclastic flow runout distances on Mars.

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.

Age of the Moncha Tundra fault, Kola Peninsula: Evidence from the Sm-Nd and Rb-Sr isotopic systematics of metamorphic assemblages

The first Sm-Nd and Rb-Sr dates were obtained for the dynamometamorphic processes associated with the origin and evolution of the Moncha Tundra fault, Kola Peninsula, which separates two large Early Paleoproterozoic layered intrusions: the Monchegorsk Ni-bearing mafic-ultramafic intrusion and the Main Range massif of predominantly mafic composition. The fault belongs to the regional Central Kola fault system, whose age was unknown. The material for the dating included metamorphic minerals from blastomylonitic rocks recovered by structural borehole M-1. Mineralogical thermobarometry suggests that the metamorphism occurred at 6.9–7.6 kbar and 620–640°C, which correspond to the amphibolite facies. The Sr and Nd isotopic systems were re-equilibrated, and their study allowed us to date the dynamometamorphic processes using mineral isochrons. It was established that the Moncha Tundra fault, and, respectively, the whole Central Kola fault system appeared in the middle of Paleoproterozoic ∼2.0–1.9 Ga, simultaneously with the Svecofennian orogen in the central part of the region and the Lapland-Kola orogen in its northeastern part. Another episode of dynamometamorphism that occurred at 1.60–1.65 Ga is envisaged.

Seismic imaging of the aftershock zone of the 2001 Mw 7.7 Bhuj earthquake, India

Detailed seismic tomography reveals that the 2001 Mw 7.7 Bhuj earthquake was associated with a 10–14% increase in Vp and Vs and a 10% decrease in Vp/Vs in the 10 to 35 km depth range covering a 2750 km2 area beneath the aftershock zone. This anomaly could be attributed to the existence of a lithological heterogeneity or a pluton of mafic composition that might have intruded during the rifting in early Jurassic (∼160 Ma). The Banni basin and the Wagad uplift are found to be associated with high velocity intrusive bodies extending from 5 to 35 km depth. A few patches of low Vp and Vs and high Vp/Vs between 10 to 30 km depth have also been detected on the causative 45° south dipping north Wagad fault (NWF) for the 2001 mainshock, which may be attributed to a fluid filled fractured rock matrix.