Lead isotope and trace element composition of K-feldspars from peraluminous granitoids of the Late Devonian South Mountain Batholith (Nova Scotia, Canada): implications for petrogenesis and tectonic reconstruction (original) (raw)
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Proceedings of the Ocean Drilling Program, 153 Scientific Results
and 923A were drilled into crust of the Mid-Atlantic Ridge near the Kane Fracture Zone (MARK) area during Leg 153 of the Ocean Drilling Program. Holes 920B and 920D were drilled into an ultramafic massif and Sites 921, 922, and 923 were drilled into a gabbroic massif, both of which are located on the western rift valley wall of the Mid-Atlantic Ridge south of the Kane Transform. Bulk-rock major-, trace-, and rare-earth element (REE) analyses of diabasic, gabbroic, and ultramafic rocks recovered from these holes are reported here. These bulk analyses are augmented by the results of mineral chemistry studies on a subset of the same samples. Large ranges in bulk-rock and mineral chemistry are documented from all rock types. Ultramafic rocks in Holes 920B and 920D are interpreted to be dominantly residual mantle, but they include variably fractionated ultramafic and mafic cumulates that have intrusive contacts with the residual mantle harzburgites. Bulk-rock major-and compatible trace-element abundances, as well as petrographic data for residual harzburgites, indicate that a fertile MORB mantle was depleted by-15% to 20% partial melting or 10%-15% if a more depleted mantle source, such as Tinaquillo Lherzolite, is chosen. The mean extent of melting is likely to have been approximately half of the maximum value computed based on the residuum. Incompatible traceelement data show, however, that this residuum may have been part of an open system and refertilized at late stages by melts flowing through a locally porous matrix and later by more channelized melts (veins) as the residuum became part of the mechanical lithosphere. The crystallization products of these late melts include disseminated magmatic clinopyroxene and narrow veins or composite veins of dunite, wehrlite, pyroxenite, and gabbroic rocks. Ultramafic vein samples are variably depleted to enriched in incompatible elements and span a wide range of fractionation extents based on bulk-rock and mineral chemistry. Melts calculated to have been in equilibrium with clinopyroxene in ultramafic and mafic samples from Site 920 vary widely. They are dominantly ultradepleted, but include some samples that are enriched in incompatible elements (Na and Ti) with respect to MARK basalts, glasses, and Leg 153 diabases. The range in composition cannot simply be explained by crystal fractionation of a single parental magma, but requires a broad range of parental melts or their derivatives to be in equilibrium with clinopyroxene. Bulk-rock and mineral chemistry studies of residual and cumulate ultramafic rocks support the notion of an open-system, near-fractional mantle melting column. The residual peridotites were also cut by late-stage, variably altered, high-MgO (13-15 wt%) diabase dikes with quenched margins. Gabbroic samples from Sites 921, 922, and 923 drilled within the gabbroic massif likewise cover a broad spectrum of lithologies and compositions, and include troctolites, olivine gabbros, gabbros, oxide gabbros, felsic diorites, and quartz diorites. Melt compositions calculated to be in equilibrium with gabbroic clinopyroxene include melts that range from those that are significantly more fractionated to less fractionated than basaltic glasses from the MARK area, but also show a smaller range of parental melts in gabbroic samples when compared to the range documented in Site 920 ultramafic and mafic samples. Hole 923A, in which recovery was high, shows clear evidence of downhole cryptic chemical variation consistent with recharge and magma mixing within subaxial magma chambers. In addition, bulk-rock REE abundances in gabbroic samples show both enriched and depleted light REE (LREE) patterns. The LREE abundances range from less than 1 X chondrite to >IOO × chondrite in gabbroic samples. MARK basaltic rocks cover a much narrower range, from 6 to 24 X chondrite. The chondritenormalized La/Yb ratios of plutonic rocks vary from 0.3 to 2.3, whereas MARK basalts have a narrower range, from 0.5 to 1.2. Extended REE patterns for plutonic rocks tend to be spiked with Sr and Eu anomalies for all samples, and both positive and negative Zr and Ti anomalies for primitive and evolved samples, respectively. The range of incompatible trace-element depletions and enrichments in plutonic samples, in part, reflects modal variations in clinopyroxene and Plagioclase, the abundance of trapped intercumulus melt, and a range of parental melt compositions input into the subaxial plumbing system. Ultramafic and plutonic rock compositions are interpreted to indicate that MORB erupted at the surface in the MARK region represents the more homogeneous hybrid derived by mixing of a diverse set of liquids generated within the mantle melting column or that evolved in plutonic environments in the crust or mantle.
Journal of Petrology
K-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/ Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial 87 Sr/ 86 Sr ratios ( I Sr ) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high I Sr -Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of I Sr , but a larger range of Rb/Sr. Lower I Sr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high I Sr silicic melts (as seen in the inner cores) and later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.
Proceedings of the Ocean Drilling Program, 153 Scientific Results
Drilling on the western wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone at MARK (23°20'N and 23°30'N) recovered plutonic rocks <l Ma in age. A selection of these rocks has been analyzed for their Sr-, Nd-, Pb-, and O-isotope compositions. O-isotope analyses of gabbro mineral separates have near primary magmatic values. δ 18 θ cpx vs. δ 18 θ plag systematics support only limited fluid-rock interaction, predominantly at moderate to low temperatures (200°-300°C). Combined Sr-and O-isotope data indicate that, even where fluids have found ingress into the gabbroic crust, they have had limited compositional consequences, presumably because seawater was rapidly modified by interaction with the overlying basalt crust. Pb-and Srisotope compositions of leachates further demonstrate that, for most samples, seawater was the principal component in hydrothermal fluids; in a few cases, however, sediments were also involved. Sr-, Nd-, and Pb-isotope systematics of leached gabbro mineral separates indicate that, to a first-order approximation, the sub-MARK mantle is isotopically homogeneous from near the middle of the spreading cell (Sites 921-924) to its boundary in the south (Site 920). Trace-element concentrations of Sr, Nd, and Pb (and, by implication, other incompatible trace elements) must also be homogeneous. Thus, variations in trace-element concentrations in gabbros and calculated parental magmas must be the product of the melting regime and subsequent fractionation processes in the magma chamber(s) and not variations in the mantle source composition. Superimposed on this homogeneity is smaller scale, within-sample, isotope disequilibrium between coexisting Plagioclase and clinopyroxene. Such heterogeneities provide evidence of complex magma chamber processes and are most likely to be preserved at the beginning of magmatic cycles. Unusual isotopically enriched compositions are found in the MARK area as diabase and amphibolitized microgabbros that crosscut the peridotite section. Restriction of these distinctive compositions in the peridotite sections at Site 920 may result from the lower magma supply rate expected near the boundary of a spreading cell. The average isotopic composition within the MARK area has changed over a period of <l m.y., becoming more radiogenic in its Pb-isotope composition but less radiogenic in its Nd composition, implying that the source is currently less depleted than that available 750,000 yr earlier.
The Central Hearne supracrustal belt (CHSB), Hearne domain, Western Churchill Province forms a large (ca. 30,000 km2), greenschist grade Neoarchaean (2711–2660 Ma) terrane of predominant metavolcanic and less common metasedimentary rocks intruded by three groups of plutonic rocks. Pre-tectonic group 1 plutons (>2690 Ma) include rare gabbro, dominant diorite to tonalite and rare granodiorite and granite. These intrude and incorporate angular inclusions of cogenetic volcanic rocks and have variably developed foliations. Syn-tectonic group 2 plutons (2690–2679 Ma) comprise rare gabbro and diorite, predominant tonalite and granodiorite and rare granite, and exhibit N–S-trending, ductiley deformed supracrustal schlieren-rich intrusive margins. Post-tectonic group 3 plutons are rare, and comprise either potassic, biotite monzogranite or alkalic nepheline syenite and rare carbonatite. Abundant tonalite to granodiorite, biotite ± hornblende – bearing mineralogies and metaluminous and generally low-medium K2O compositions, indicate that most rocks are I- or M-type granitoids. Molecular Na–Ca–K variations and AFM indices indicate transitional calc-alkaline – trondjhemitic trends with both tholeitic and calc-alkaline affinities. Rare earth and incompatible element variations suggest that most granitoids exhibit volcanic arc- or tonalite–trondhjemite–granodiorite (TTG)-like abundances with multi-element patterns varying gradationally from La-poor (LaN/YbN <12) to La-rich compositions (LaN/YbN > 12). Nd isotopic values overlap with contemporaneous depleted mantle indicating that the granitoids represent melts derived from predominantly juvenile mantle or crust. La-poor rocks likely formed through low-P anatexis of amphibolitic crust with plagioclase + amphibole ± clinopyroxene present, whereas high-La rocks were generated via high-P partial melting of a garnet+ clinopyroxene-bearing protolith (plausibly a subducted slab). Granitoid evolution from early dominant low-P, tholeiitic and calc-alkaline melts, to later, predominant high-P, high-Al2O3 TTG melts, reflects a change in the tectonomagmatic setting at ca. 2690 Ma. Construction of proto-arc crust from ca. 2711–2690 Ma, and extension of the leading edge of normal (ca. 40 km) Archaean oceanic crust, in response to lithospheric processes analogous to those of the Eocene SW Pacific Ocean, resulted in asthenospheric upwelling, intrusion of mantle-derived melts into the lower crust, and their subsequent ascent and fractionation. Partial melting at the base of the extended, thick oceanic crust likely yielded sparse high-La melts at this time. At ca. 2690 Ma, a change from an extensional to a shortening regime resulted in initiation of subduction of adjacent oceanic lithosphere, partial melting of the eclogitic downgoing slab and generation of voluminous, high-P, La-rich granitoid magmas. This yielded less abundant low-P, La-poor melts emplaced into the “infant-arc” crust during and following tectonism. The complete sequence was intruded by late potassic granites derived from tonalitic lower crust and alkalic magmas that originated in the lithospheric mantle.
Chemical Geology, 2009
We present Pb isotope and microstructural studies of deformed K-feldspar megacrystic granodiorites in the westernmost part of the Karelian Craton near the Archean-Proterozoic boundary zone in east-central Finland. Our studies place constraints on the timing and the temperature and strain rate conditions of the deformation. The Pb isotope evidence indicates that U was decoupled from Th approximately at the time of crystallization, which generated high whole-rock Th/U ratios. With time, high 208 Pb/ 204 Pb ratios were developed at high Th/U domains of the rock. Observed correlation between 208 Pb/ 204 Pb ratios and deformation microstructures of K-feldspar megacrysts suggests a later deformation-induced Pb isotope redistribution between K-feldspars and whole rock. The coupled 238 U-206 Pb and 235 U-207 Pb systematics show that this Pb isotope exchange occurred at ca. 1.9 Ga concurrently with the peak of the Svecofennian orogeny. Our Th-Pb and U-Pb model age calculations for K-feldspar-whole rock pairs indicate different degrees of incomplete equilibration that correlates with the degree of deformation. The microstructures of K-feldspars indicate low-to medium-T conditions (400-500°C) and variable strain rates for deformation. We conclude that Pb isotope compositions and microstructures of K-feldspars provide valuable constraints on assessing Proterozoic imprints in Archean rocks.
Atlantic Geology, 1991
In order to evaluate the late-stage physical and chemical evolution of the Upper Devonian, peraluminous South Mountain Batholith, mineral chemistry (muscovite, feldspars), oxygen isotopes (whole rock, mineral separates) and Al/Si ordering of Kfeldspar have been investigated. Muscovite, K-feldspar and albite define coherent chemical trends with respect to compatible (B a, Sr, Eu) and incompatible (Li, Cs, Rb) elements such that crystal fractionation processes are reflected. However, the influence of a fluid phase is seen in terms of Eu/Eu* of greisen muscovite and the light REE profiles (chondritic) of chemically evolved K-feldspar. The dominance of monoclinic K-feldspar, even in pegmatitic environments, indicates that either single or multiple processes impeded Al/Si ordering (e.g., cooling rate, fluid and/or rock composition). However, locally the development of triclinic K-feldspar dominates where intense fluid-rock interaction occurred (e.g., greisens), thus reflecting alteration at temperatures <450"C. 5lsO values of minerals from pegmatites reflect disequilibrium conditions related to variable degrees of fluid-rock interaction after crystallization. Calculated 81 * 0^ (mostly 5-10 %>) indicates a dominantly magmatic fluid reservoir; bo th 1 ®Q-enriched (to +18 %») and-depleted (to <+4 %<>) fluids infiltrated the system, indicating that local fluid mixing occurred. Dans le but d'6valuer Involution physique et chimique dans ses demiers stades du batholite peralumineux devonien superieur de South Mountain, on a 6tudie la chimie des min£raux (muscovite, feldspaths), les isotopes de l'oxygene (sur roche totale et sur fractions monominerales) et l'ordonnance Al/Si au sein des feldspaths potassiques. La muscovite, le feldspath potassique et l'albite d&erminent des tendances chimiques coh&entes visa -vis des elements compatibles (Ba, Sr, Eu) et incompatibles (Li, Cs, Rb), de fa5on telle qu'elles refletent des processus de cristallisation fractionnee. Cependant, l 'influenee d 'une phase fluide s 'exprime en termes du rapport Eu/Eu* des muscovites dans les greisens et des profils (chondritiques) des T.R. legeres de feldspaths potassiques ayant subi une Evolution chimique. La dominance du feldspath potassique monoclinique, meme dans les environnements pegmatitiques, indique qu'un ou plusieurs processus firent entrave a l'ordonnance Al/Si (e.g., taux de refroidissement, composition du fluide et/ou de la roche). Cependant, d 'une faijon locale, le developpement de feldspath potassique triclinique domine la oil l 'int^raction entre le fluide et la roche fut intense (e.g., dans les greisens), refletant ainsi une alteration a des temperatures atteignant 450°C. Le dosage 81 ®0 des mineraux preleves dans les pegmatites reflete des conditions de desequilibre reliees a divers degres d 'interaction entre le fluide et la roche apres leur cristallisation. La valeur calcuiee 81 * 0^^ (pour la plupart 5-10 %<>) indique un reservoir de fluide a dominance magmatique; le systeme fut infiltre par des fluides aussi bien enrichis (jusqu'k +18 %») qu'appauvris (jusqu'a <+4 %o), ce qui indique qu'un melange de fluides se produisit a I'echelle locale.
Canadian Journal of Earth Sciences, 1998
Middle Ordovician felsic magmatism contemporaneous with Bathurst Camp Pb-Zn volcanogenic massive sulphide (VMS) deposits consists of strongly altered volcanic to subvolcanic rocks, belonging to the Tetagouche Group, and relatively unaltered granitoid plutons, which are divided into northern, central, and southern groups within the Miramichi Highlands. Calc-alkalic felsic volcanic rocks and northern plus central plutons have ε Nd (T) values ranging from -8.2 to -1.9 and -4.0 to +0.3, respectively. They exhibit within-plate-type volcanic and transitional I-to A-type granite geochemical characteristics. Granitoid rock δ 18 O values range from +8.0 to +10.1‰. Published granitoid rock Pb isotopic compositions overlap unpublished galena data from Bathurst VMS deposits. Field, geochemical, and isotopic evidence indicate that these volcanic and granitoids rocks are consanguineous and mainly derived from Proterozoic or older infracrustal sources. Alkalic felsic volcanic rocks, and associated alkaline basaltic rocks, are more juvenile (ε Nd (T) = +3.2 to +4.2) and were possibly derived from slightly enriched mantle sources. Southern plutons exhibit continental arc-type features. The felsic magmatism and VMS deposits likely formed in an Okinawa-type back-arc basin developed from rifting the Early Ordovician Popelogan continental arc, of which the southern plutons are remnants. Correlations between pluton groups and volcanic formations indicate that felsic magmatism was erupted through and onto the Miramichi Group. As most felsic volcanic formations lack plutonic equivalents, the Tetagouche Group probably does not represent disrupted slices of an originally conformable stratigraphic section. This supports a model in which thrust slices juxtapose remnants of volcanic centres erupted at different locations within a back-arc basin.
Recent work in the northeastern Kisseynew Domain has provided evidence that this area is underlain by or intercalated with Superior Province basement that is exposed in structural culminations. Where examined, this basement is mantled by Ospwagan-like paragneiss that may be an exploration target for Thompson-type Ni deposits. Closely associated with the basement culminations is the ca. 1885 Ma K-feldspar–porphyritic Footprint Lake plutonic suite (FLPS), which ranges in composition from syenite–monzonite to syenogranite–monzogranite. It represents a rare alkalic to alkali-calcic suite, low-silica end members of which are thought to represent lowdegree partial melts of subduction-modified subcontinental lithospheric mantle (SCLM). Felsic end members are thought to reflect mixing between SCLM-derived magma and crustal melt. Archean Nd-model ages obtained from low-silica FLPS samples are interpreted as reflecting their mantle sources, not crustal contamination, and as such support the p...
Petrologic evidence for K-feldspar metasomatism in granulite facies rocks
Chemical Geology, 1998
We present evidence for K-feldspar metasomatism in charnockitic granulites from two well-known terranes: the Shevaroy Hills Massif, S. India (750°C, 8 kbars) and the Bamble Sector, S.E. Norway (790°C, 7.5 kbars) in the form of K-feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of K-feldspar in an uneven scattering of