An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines

Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and ¹⁸O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source.     

Noble gases and chemical composition of Shergotty mineral fractions,Chassigny, and Yamato 793605: The trapped argon-40/argon-36 ratio and ejection times of Martian meteorites

Abstract— We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The ⁴⁰K-⁴⁰Ar isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a ⁴⁰Ar/³⁶Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped ⁴⁰Ar/³⁶Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped ⁴⁰Ar/³⁶Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain ³He-, ²¹Ne-, and ³⁸Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605, 4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars ～1 Ma before the shergottites. Chassigny yields the previously known ejection age of 11.6 Ma.     

Spatial variability of watermass conditions within the European Epicontinental Seaway during the Early Jurassic (Pliensbachian–Toarcian)

In order to constrain spatial variability in watermass conditions within the European Epicontinental Seaway prior to, during and after the Toarcian Oceanic Anoxic Event, carbon (δ¹³C_bel, δ¹³C_carb) and oxygen (δ¹⁸O_bel, δ¹⁸O_carb) isotope records were obtained from three sections in the Grands Causses Basin (southern France). These data were then compared with similar records along a north–south transect across the European Epicontinental Seaway. As the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes, an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite. Intra‐rostral carbon and oxygen‐isotope data from six belemnite specimens belonging to the genus Passaloteuthis were collected. Intra‐rostral carbon‐isotopes are influenced by vital effects, whereas oxygen‐isotopes reflect relative changes in temperature and salinity. Palaeotemperatures calculated from δ¹⁸O_bel‐isotope records from the Grands Causses Basin confirm relatively low temperatures throughout the Late Pliensbachian. Similar cool water conditions have previously been shown in Germany, England, Spain and Portugal. A temperature increase of up to 6 °C is observed across the Pliensbachian–Toarcian boundary. A pronounced negative shift of at least ?3‰ (Vienna‐Pee Dee Belemnite) is recorded in bulk carbonate carbon during the lower Harpoceras serpentinum zone, typical of the Toarcian Oceanic Anoxic Event. Before and after the Toarcian Oceanic Anoxic Event, a good correlation between δ¹³C_carb and δ¹³C_bel exists, indicating well‐ventilated bottom‐waters and normal marine conditions. Instead, data for the Toarcian Oceanic Anoxic Event indicate the development of a strong north–south gradient in salinity stratification and surface‐water productivity for the Western Tethyan realm. This study thus lends further support to a pronounced regional overprint on carbon and oxygen‐isotope records in epicontinental seaways.     

Sequence stratigraphy in the context of rapid regional uplift and high‐amplitude glacio‐eustatic changes: the Pleistocene Cutro Terrace (Calabria,southern Italy)

The Cutro Terrace is a mixed marine to continental terrace, where deposits up to 15 m thick discontinuously crop out in an area extending for ca 360 km² near Crotone (southern Italy). The terrace represents the oldest and highest terrace of the Crotone area, and it has been ascribed to marine isotope stage 7 (ca 200 kyr bp ). Detailed facies and sequence‐stratigraphic analyses of the terrace deposits allow the recognition of a suite of depositional environments ranging from middle shelf to fluvial, and of two stacked transgressive–regressive cycles (Cutro 1 and Cutro 2) bounded by ravinement surfaces and by surfaces of sub‐aerial exposure. In particular, carbonate sedimentation, consisting of algal build‐ups and biocalcarenites, characterizes the Cutro 1 cycle in the southern sector of the terrace, and passes into shoreface and foreshore sandstones and calcarenites towards the north‐west. The Cutro 2 cycle is mostly siliciclastic and consists of shoreface, lagoon‐estuarine, fluvial channel fill, floodplain and lacustrine deposits. The Cutro 1 cycle is characterized by very thin transgressive marine strata, represented by lags and shell beds upon a ravinement surface, and thicker regressive deposits. Moreover, the cycle appears foreshortened basinwards, which suggests that the accumulation of its distal and upper part occurred during forced regressive conditions. The Cutro 2 cycle displays a marked aggradational component of transgressive to highstand paralic and continental deposits, in places strongly influenced by local physiography, whereas forced regressive sediments are absent and probably accumulated further basinwards. The maximum flooding shoreline of the second cycle is translated ca 15 km basinward with respect to that of the first cycle, and this reflects a long‐term regressive trend mostly driven by regional uplift. The stratigraphic architecture of the Cutro Terrace deposits is the result of the interplay between regional uplift and high amplitude, Late Quaternary glacio‐eustatic changes. In particular, rapid transgressions, linked to glacio‐eustatic rises that outpaced regional uplift, favoured the accumulation of thin transgressive marine strata at the base of the two cycles. In contrast, the combined effect of glacio‐eustatic falls and regional uplift led to high‐magnitude forced regressions. The superposition of the two cycles was favoured by a relatively flat topography, which allowed relatively complete preservation of stratal geometries that record large shoreline displacements during transgression and regression. The absence of a palaeo‐coastal cliff at the inner margin of the terrace supports this interpretation. The Cutro Terrace provides a case study of sequence architecture developed in uplifting settings and controlled by high‐amplitude glacio‐eustatic changes. This case study also demonstrates how the interplay of relative sea‐level change, sediment supply and physiography may determine either the superposition of cycles forming a single terrace or the formation of a staircase of terraces each recording an individual eustatic pulse.     

Tectonic control on peritidal carbonate parasequence formation: an investigation using forward tectono-stratigraphic modelling

Studies of Quaternary extensional faults indicate that they have instantaneous amounts of throw (0·4 to 4 m), average slip rates (0·05 to 2·8 m kyr⁻¹) and frequency of recurrence (<40 000 years) accounting for the accommodation space required for the accumulation of peritidal carbonate parasequences (PCPs). Hangingwall sites and graben are characterized by fault down-dropping together with regional subsidence, and footwall sites and horsts by fault-related uplift alternating with periods of regional subsidence. The relative sea-level curves generated by these processes operating in a maritime rift setting are used as inputs to a forward stratigraphic modelling program SedTec2000 to simulate how fault-related changes in accommodation space can account for high-frequency PCP formation. Each instantaneous fault slip generates a flooding surface or aggradation in hangingwall and graben settings. High-frequency cycles in hangingwall sites are either symmetric (deepening then shallowing upward) or asymmetric (shallowing-upward). The major factor controlling cycle types is the balance between rates of carbonate accumulation and generation of accomodation space. High-frequency cycles in footwall sites and horsts comprise shallow subtidal facies, with no distinctive bathymetric trends, capped by erosional boundaries generated by footwall uplift. The modelled cycles are of the same thickness, with bathymetric trends and frequency to cycles commonly interpreted to be due to orbitally driven eustatic sea-level changes or autocyclic processes. These numerical experiments demonstrate that high-frequency PCPs can be generated by tectonic, fault-related processes, a hypothesis that is frequently discounted.