A. Krishna Sinha

Strontium isotopic and selected trace element variations between two Aleutian volcanic centers (Adak and Atka): implications for the development of arc volcanic plumbing systems

Major and trace element concentrations and initial 87Sr/86Sr ratios of lavas from the Aleutian volcanic centers of Adak and Atka have been used to study the evolution of their respective lithospheric plumbing systems. The centers are within 150 km of one another and show similar overall silica ranges (47–67%), but Adak (∼40 km3) is smaller than Atka (∼200 km3). Adaks lavas are chemically and isotopically heterogeneous (87Sr/86Sr:0.70285–0.70330) and two units contain lithospheric xenoliths. The lavas of the much larger Atka, on the other hand, have much less variability in major and trace elements as well as 87Sr/86Sr (0.70320–0.70345). We suggest that these characteristics are a measure of the relative maturity and “cleanliness” of the lithospheric plumbing systems that supply magma to these centers. Because Aleutian volcanic centers often remain fixed for relatively long periods of time (∼5 m.y.), once established, magmatic passageways are repeatedly used. Young plumbing systems are relatively cool and contain large amounts of wallrock contaminant, and ascending magmas undergo contamination as well as concurrent crystallization and fractionation. With time, however, heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas. In effect, the plumbing system matures. The chemical heterogeneity displayed by young, “dirty” systems (like Adak) reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation. Thus, it is the petrologic data of mature, clean systems, like Atka, that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution.

Geoinformatics: Transforming data to knowledge for geosciences

An integrative view of Earth as a system, based on multidisciplinary data, has become one of the most compelling reasons for research and education in the geosciences. It is now necessary to establish a modern infrastructure that can support the transformation of data to knowledge. Such an information infrastructure for geosciences is contained within the emerging science of geoinformatics, which seeks to promote the utilization and integration of complex, multidisciplinary data in seeking solutions to geoscience-based societal challenges.

Physical and chemical response of zircons to deformation

An investigation of U-Pb isotopic systematics in zircons from mylonitized Henderson Gneiss (Sinha and Glover 1978) revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic Pb at ∼ 460 m.y. To further investigate the relationship between Pb loss, U gain, and grain size reduction associated with increasing strain in the shear zone, we have characterized the chemistry and morphology of zircons in the mylonitic rocks, using both electron microprobe analysis and scanning electron microscopy.SEM photographs of the zircons indicate that strain-correlated fracturing and size reduction of the zircons accompanied Pb loss throughout the mylonite zone. Stresses imposed by the expansion of initially U-rich, α-damaged portions of the crystal resulted in microfracturing of the more brittle crystalline material proximal to the U-rich zones. During mylonitization, fractures propagated preferentially along these zones allowing metamorphic fluids to penetrate the easily-leached, α-damaged portions of the zircons. Removal of ∼75% of the radiogenic Pb from zircons in the least-deformed zone of the mylonites may have occurred via this mechanism.Irregular, porous zircon overgrowths are also evident from the SEM photographs. Overgrowths are strongly enriched in U, Y and P with respect to the relict, Henderson Gneiss-derived cores, and tend to increase in volume from the protomylonite to the blastomylonite. Thus, the development of overgrowths on the zircons accounts for the U gain observed by Sinha and Glover (1978), and indicates that the transport of high field strength cations (e.g., Zr4+, Hf4+, U4+, etc.) occurred during prograde mylonitization at 460 m.y.A retrograde shearing event at ∼ 273 m.y. caused no further disturbance in the U-Pb isotopic systematics of the zircons. Pb retention by zircons during the later episode may have been the result of 1) the participation of H2O-rich, relatively noncorrosive fluids and/or 2) the lack of further fracturing and size reduction in a strain gradient of lower magnitude than the prograde event.

Fluid interaction and element mobility in the development of ultramylonites

ABSTRACT Shear zones are often characterized by the presence of mylonitic fabrics. The textural development of such fabrics is enhanced by the presence of a fluid phase. In a single specimen of rock from the Brevard fault zone in North Carolina, we can demonstrate the development of ultramylonite domain by focused fluid flow. The ultramylonite interface retains a sharp textural and chemical discontinuity; this suggests that solute transport was dominantly parallel to the tectonic layering. Major chemical changes between the ultramylonite zone and the protolith include losses of Si(>2, Na 2 0, and K 2 0 and gain of CaO, FeO, and HjO in the high-strain domain. INTRODUCTION During the tectono-thermal evolution of mountain belts, zones of high strain are often recorded by the development of mylonites. Although mylonites and subcategories within mylonites (Sibson, 1977) can be generated by numerous well-documented physical processes, their chemi-cal and isotopic evolution is much more complex. Mechanical processes often associated with the development of high-strain zones include micro-fracturing, dilatancy, grain-boundary sliding, and dislocation glide. These properties are strongly dependent on the modal mineralogy (bulk compo-sition), heterogeneity of the rock, and the temperature and pressure con-ditions during deformation. Such variations in the mechanics of deforma-tion have led to the more common classification of shear zones as either ductile or brittle (Sibson, 1977; Ramsay, 1980). However, most shear zones are characterized by the availability of fluids that cause a drastic change in the style of deformation by altering the behavior and assem-blage of minerals (hydration reactions) and the rates of chemical and mechanical processes during progressive deformation. The presence of a fluid phase during deformation enhances solution transport, microfractur-ing, and recrystallization Suc.h chemically active zones generate mineral assemblages that record the metamorphic history of the mylonites, al-though it is not unusual to find minerals that persist through such an event. These relict minerals, if recognized, can identify equilibrium vs. nonequilibrium assemblages observed in shear zones. Clearly, the pres-ence of relict minerals would have a substantial impact on the trace-element and isotopic signature of the rock. To more carefully document the behavior of elements in a strain zone that has abundant fluid, we have chosen to study a transect across the Brevard fault zone, near Rosman, North Carolina. Within this tran-sect, one particular outcrop provided a single large specimen that over a distance of-25 cm showed the entire strain-gradient fabric from proto-mylonite to ultramylonite (see Sibson, 1977, for mylonite series) ob-served by us as discontinuous patches in the longer (15-km) transect. We report petrographic, textural, and chemical data from this block and pro-vide a physical model to explain these data.

The effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis: A model

Abstract The initial Pb isotopic composition of anatectic granites is modelled as a mixture of Pb derived from major minerals that have characteristically low 238 U 204 Pb and 232 Th 204 Pb ratios (i.e., feldspars, biopyroboles, etc.) with Pb contributed from dissolved accessory minerals with characteristically high 238 U 204 Pb and 232 Th 204 Pb ratios (i.e., zircon, monazite, etc.). The initial Pb isotopic composition of anatectic granites is a function of (a) the age, type, modal distribution and heterogeneity in the initial U and Th content of the accessory minerals, (b) variation in melt composition and temperature during anatexis (c) the fraction of the source melted, and (d) the extent to which the melt was homogenized prior to crystallization. The differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the Pb isotopes from other chemical and isotopic systems in granite rocks. Granitic plutons derived by anatexis of the same source material are not required to have the same initial Pb isotopic composition. Low temperature, peraluminous granites are more likely to exhibit homogeneous initial Pb isotopic compositions dominated by the Pb contained within the major mineral assemblage of the source material. In contrast, the initial Pb isotopic composition of higher temperature, metaluminous or peralkaline anatectic granites is more likely to be heterogeneous, unless the melts are homogenized during emplacement, and will record a larger fraction of the radiogenic Pb component contained in the accessory mineral assemblage of the source material. Heterogeneity in the initial Pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic Pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis. The use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites.

Geochemical and strontium isotopic characteristics of parental Aleutian arc magmas: evidence from the basaltic lavas of Atka

Eighteen flows from a basal stratigraphic sequence on the Aleutian Island of Atka were analyzed for major elements, trace elements and initial 87Sr/86Sr ratios. Petrographically, these lavas contain abundant plagioclase (24–45%) and lesser amounts of olivine (<7%), magnetite and clinopyroxene phenocrysts. Compositionally, the lavas are high-alumina (∼20wt%) basalts (48–51 wt% SiO2) with low TiO2 (<1%) and MgO (<5%). Within the section, compositional variations for all major elements are quite small. While MgO content correlates with olivine phenocryst contents, no such relationship exists between the other oxides and phenocryst content. These lavas are characterized by 8–10 ppm Rb, high Sr (610–669 ppm), 308–348 ppm Ba and very constant Zr (23–29 ppm) and Sc (23–29 ppm) abundances. Ni and Cr display extremely large compositional ranges, 12–118 ppm and 12–213 ppm, respectively. No correlation exists between trace element concentrations and phenocryst contents. Strontium isotopic ratios show a small but significant range (0.70314–0.70345) and are slightly elevated with respect to typical MORB. No systematic correlation between stratigraphic position and petrography or geochemistry is evident. REE abundances measured on six samples are LREE enriched ((La/ Yb)N = 2.20–2.81) and display similar chondrite normalized patterns. One sample has a slight positive Eu anomaly but the other lavas do not. Compared to other Aleutian basalts of similar silica content, these lavas are less LREE enriched and have lower overall abundances. The geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature. Published phase relations indicate fractionation of a more MgO-rich magma could not have produced these lavas. The high Al2O3 and low MgO and compatible element abundances suggest a predominantly oceanic crustal source for parental high-alumina basalts.

U/Pb Systematics of zircons during dynamic metamorphism

Eight zircon fractions from the Henderson Gneiss were analyzed from the Brevard mylonites and adjacent Inner Piedmont rocks near Rosman, North Carolina, to determine variations of U/Pb systematics of zircons in a pressure-dominated metamorphism. Substantial uranium gain occurred during zircon recrystallization and size reduction.At Rosman, the Henderson Gneiss zircons probably formed about 600 m.y. ago and now show evidence of being binary mixtures. They experienced mylonitization at about 450 m.y. ago during Taconic deformation and metamorphism in the almandine amphibolite facies. The lower intensity, upper greenschist or greenschist-amphibolite Acadian (360–390 m.y.) metamorphism and mylonitization (recrystallization) had little effect on the zircons but disturbed Rb/Sr systems.The ages of dynamic metamorphism and thrusting (450, 360–390) in the Piedmont and Blue Ridge near and along the Brevard zone at Rosman, North Carolina, agree well with the tectonic interpretations of the Valley and Ridge Taconic (ca. 470-400 m.y.) and Acadian (ca. 375-330 m.y.) clastic wedges. Intermittent thrusting, folding, and uplift rather than continuous secular uplift of the crystalline terrane seems indicated.

Differential response of zircon U−Pb isotopic systematics to metamorphism across a lithologic boundary: an example from the Hope Valley Shear Zone, southeastern Massachusetts, USA

AbstractA detailed morphological, chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late Precambrian Ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on U-Pb isotopic systematics. Late Paleozoic, amphibolite-grade (approx. 600°C) mylonitization of the Ponaganset gneiss at this locality is associated with movement along the Hope Valley Shear Zone. The response of zircon to metamorphism in each gneiss unit is distinct: zircons in gray augen gneiss are uncorroded and not overgrown, whereas zircons from fluorite-bearing pink granitic gneiss are variably corroded and over 50% bear opaque overgrowths. The zircon overgrowths are chemically distinct from the primary cores, and contain high conentrations of Hf, U, HREE, and Th. Mylonite derived from the gray gneiss contains only a small population of Hf-U-rich metamorphic zircon, but zircons in the pink gneiss-derived mylonite are dominated by the Hf-U-rich metamorphic component. In terms of their U-Pb isotopic systematics, overgrowth-free zircons from both units are markedly discordant (gray, 10–20%, pink, 35%), but overgrown zircons from the pink gneiss are up to 70% discordant. Zircons from the mylonites yield younger Pb−Pb and U−Pb ages than those of the protolith gneisses, and isotopic data from each gneiss + mylonite pair define a linear array on concordia plots. Upper intercept ages of the gray gneiss (621+/−27 Ma) and the pink gneiss (635+/−50 Ma) indicate that the crystallization of both units was coeval, and the lower intercept ages (gray, 270+/−92 Ma; pink, 285+/−26 Ma) fall within the range of other published age estimates for Alleghanian metamorphism in southeastern New England (e.g., Zartman et al. 1988). New growth of zircon suggests that Zr was mobile during metamorphism. The presence of fluorite in the pink gneiss, and a discontinuity in log

A Detailed Pb Isotopic Study of Crustal Contamination Assimilation : The Edgecumbe Volcanic Field, SE Alaska

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