Comparison of thermal modeling, microstructural analysis, and T i‐in‐quartz thermobarometry to constrain the thermal history of a cooling pluton during deformation in the M ount A bbot Q uadrangle, CA
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Granitic plutons commonly preserve evidence for jointing, faulting, and ductile fabric development during cooling. Constraining the spatial variation and temporal evolution of temperature during this deformation could facilitate an integrated analysis of heterogeneous deformation over multiple length-scales through time. Here, we constrain the evolving temperature of the Lake Edison granodiorite within the Mount Abbot Quadrangle (central Sierra Nevada, CA) during late Cretaceous deformation by combining microstructural analysis, titanium-in-quartz thermobarometry (TitaniQ), and thermal modeling. Microstructural and TitaniQ analyses were applied to 12 samples collected throughout the pluton, representative of either the penetrative “regional” fabric or the locally strong “fault-related” fabric. Overprinting textures and mineral assemblages indicate the temperature decreased from 400–500°C to <350°C during faulting. TitaniQ reveals consistently lower Ti concentrations for partially reset fault-related fabrics (average: 12 ± 4 ppm) than for regional fabrics (average: 31 ± 12 ppm), suggesting fault-related fabrics developed later, following a period of pluton cooling. Uncertainties, particularly in TiO2 activity, significantly limit further quantitative thermal estimates using TitaniQ. In addition, we present a 1-D heat conduction model that suggests average pluton temperature decreased from 585°C at 85 Ma to 332°C at 79 Ma, consistent with radiometric age data for the field. Integrated with the model results, microstructural temperature constraints suggest faulting initiated by ∼83 Ma, when the temperature was nearly uniform across the pluton. Thus, spatially heterogeneous deformation cannot be attributed to a persistent temperature gradient, but may be related to regional structures that develop in cooling plutons.Lineation
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Abstract The Mourne Mountains magmatic center in Northern Ireland consists of five successively intruded granites emplaced in the upper crust. The Mourne granite pluton has classically been viewed as a type locality of a magma body emplaced by cauldron subsidence. Cauldron subsidence makes space for magma through the emplacement of ring dikes and floor subsidence. However, the Mourne granites were more recently re-interpreted as laccoliths and bysmaliths. Laccolith intrusions form by inflation and dome their host rock. Here we perform a detailed study of the deformation in the host rock to the Mourne granite pluton in order to test its emplacement mechanism. We use the host-rock fracture pattern as a passive marker and microstructures in the contact-metamorphic aureole to constrain large-scale magma emplacement-related deformation. The dip and azimuth of the fractures are very consistent on the roof of the intrusion and can be separated into four steeply inclined sets dominantly striking SE, S, NE, and E, which rules out pluton-wide doming. In contrast, fracture orientations in the northeastern wall to the granites suggest shear parallel to the contact. Additionally, contact-metamorphic segregations along the northeastern contact are brecciated. Based on the host-rock fracture pattern, the contact aureole deformation, and the north-eastward–inclined granite-granite contacts, we propose that mechanisms involving either asymmetric “trap-door” floor subsidence or laccolith and bysmalith intrusion along an inclined or curved floor accommodated the emplacement of the granites and led to deflection of the northeastern wall of the intrusion.
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Neoproterozoic magmatism in the Hannan region at the northwestern margin of the Yangtze Block is characterized by numerous felsic plutons associated with minor mafic-ultramafic intrusions. The felsic plutons are either adakitic or normal-arc granitic in composition. The adakitic plutons are ∼735 Ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust. Among the normal-arc-related felsic plutons, the Tianpinghe pluton is the largest and has a SHRIMP zircon U-Pb age of 762 ± 4 Ma, older than the adakitic plutons in the region. Rocks from the Tianpinghe pluton have relatively high SiO (67.1–70.1 wt%) and KO + NaO (7.8–8.6 wt%) and relatively low MgO (0.7–1.3 wt%) and AlO contents (14.5–15.6 wt%), with AlO/(CaO + KO + NaO) (A/CNK) values ranging from 0.95 to 1.08. They have arc-affinity trace-element compositions that are characterized by enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with strong positive Pb and negative Ti anomalies. They have a narrow range of εNd values (+0.15 to -1.76) and relatively high zircon εHf values (+0.6 to +8.3). These geochemical features are typical of I-type granites. The rocks from the Tianpinghe pluton have relatively young single-stage and two-stage Hf model ages (1.01–1.31 and 1.31–2.01 Ga, respectively), suggesting that the pluton was generated by partial melting of newly formed basaltic rocks. On the basis of its arc-related geochemical affinity and its emplacement before voluminous adakitic magmatism but after mafic-ultramafic intrusions, the Tianpinghe pluton is considered to be Neoproterozoic arc granite formed during a period of crustal growth and reworking. Generation of the later adakitic plutons suggests that the crustal thickness increased to more than 50 km by mafic magma underplating.
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U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373+/-3 Ma, interpreted as the time of intrusion. U-Pb analyses of euhedral zircons with thick rims overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370+/-3 and 628+/-33 Ma (Larrys River and Halfway Cove plutons), 372+/-3 and approximately 660 Ma (Shelburne pluton), and 373+/-2 and approximately 732 Ma (Barrington Passage pluton). The upper intercepts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovician Meguma Group. Other zircons in the granites are generally irregular-euhedral with thin rims, and most U-Pb isotopic analyses fall between two chords from 373-2040 and 373-2300 Ma, with a few lying outside this field. These zircons are probably derived from the country rock (Goldenville Formation), which a previous study has shown contains detrital zircons with concordant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discordant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approximately synchronous with intrusion of mafic rocks and lamprophyre dikes and regional low-pressure metamorphism and was followed by rapid denudation of 5-12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates through the subducting plate to melt the overriding continental plate. Subsequent northward migration of the plume could explain both the approximately 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin.
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Research Article| May 01, 1980 Petrology of the Castle Crags pluton, Klamath Mountains, California Walter R. Vennum Walter R. Vennum 1Department of Geology, Sonoma State University, Rohnert Park, California 94928 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (5_Part_II): 1332–1393. https://doi.org/10.1130/GSAB-P2-91-1332 Article history received: 19 Mar 1979 rev-recd: 24 Oct 1979 accepted: 30 Nov 1979 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Walter R. Vennum; Petrology of the Castle Crags pluton, Klamath Mountains, California. GSA Bulletin 1980;; 91 (5_Part_II): 1332–1393. doi: https://doi.org/10.1130/GSAB-P2-91-1332 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Castle Crags pluton intrudes mafic and ultramafic rocks of the Trinity ophiolite complex in the Klamath Mountains of northern California. Contacts between the three concentrically arranged rock types which compose this 30 km2 epizonal stock are gradational. The outermost unit is a fine-grained granodiorite which has a chilled intrusive contact against the adjacent country rocks. The bulk of the pluton is a porphyritic sodic granodiorite which is characterized by euhedral potassium feldspar megacrysts. The core is occupied by a fine-grained alkalic trondhjemite. The pluton is intruded by a genetically related series of hypabyssal dikes, a series of porphyritic trondhjemitic dikes, and a suite of remarkably fresh spessartite lamprophyre dikes. Smooth variation in modes, major and trace elements, specific gravity, and porosity occurs from the margin to the core of the pluton.The Castle Crags pluton represents a single intrusion with an original composition approximating that of the chilled granodiorite. After it was chilled, the remaining magma crystallized inward from its margins. The concentric chemical variation resulted from fractional crystallization of the alkali-rich parent magma toward a residuum with mild peralkaline tendencies and was aided by inward transfer of alkalis through a volatile phase. Abundant miarolitic cavities in the inner half of the pluton indicate that second boiling occurred when the magma was approximately two-thirds crystallized. The fine-grained textures of the trondhjemite resulted from pressure quenching that accompanied escape of volatiles from the pluton. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.
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