The Rodeo de la Bordalesa Tonalite Dykes as a Lower Devonian Magmatic Event: Geochemical and Isotopic Age Constraints
Carlos Alberto CingolaniEduardo Jorge LlambíasMiguel Ângelo Stipp BaseiNorberto Javier UrizFarid ChemalePaulina Abre
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Batholith
Devonian
Outcrop
Late Devonian extinction
U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.
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Abstract The Tuolumne Batholith (TB), Sierra Nevada Batholith (USA), is an archetypal large, zoned arc intrusion ( c . 1200 km 2 ). Previous work proposed that compositional zonation observed in the TB was produced in-situ by inward differentiation of a large magma chamber and/or large-scale, intrachamber magma mixing. Recent geochronology shows that the TB was intruded over 8–9 Ma, making single pulse fractionation or mixing in a magma chamber of TB dimensions unlikely. We examine processes responsible for compositional variation in the Cathedral Peak Granodiorite, which is the largest mapped unit of the TB. New field, geochemical and geochronological work along a roughly contact-perpendicular 5 km transect indicates: (1) magmatic foliation is steeply-dipping (>60°); (2) field evidence for repeated separation of crystals from melt and local magma mixing is observed; (3) U–Pb zircon ages at opposing ends of the transect are indistinguishable within error ( c . 87.5 Ma); (4) bulk composition varies only modestly but trace elements show variable degrees of scatter; (5) εNd(t) and 87 Sr/ 86 Sr(i) have small variation compared with that in the whole TB. Geochemical and isotopic data are compatible with fractionation of major silicates and accessory minerals. However, the geochemical spatial variation, minor isotopic variation and field evidence suggest that fractionation was highly disorganized and also involved mixing with new input magma and remobilization of crystal mush as the pluton solidified. Our observations are consistent with the construction of a large and dynamic magma system within the last c . 1 Ma of TB growth.
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Rather than being inherited as detrital grains, many of the rounded and/or embayed cores in zircon crystals from granitic rocks are probably antecrysts, formed early in the magmatic systems. Using laser-ablation, sector-field ICP-MS, we obtained internally consistent weighted-mean concordia ages of 375.3 ± 2.5 Ma and 376.9 ± 2.6 for early crystallisation of two samples from the previously undated Mount Disappointment pluton in the Melbourne Zone. Within uncertainty, our date of 376.4 ± 2.4 Ma for the Baringhup pluton of the Harcourt batholith is the same as the published and our new date for the Mount Alexander pluton, in the same batholith. Using the same techniques, we produced an improved date of 370.9 ± 6.5 Ma for the I-type Ercildoun Granite in the neighbouring Bendigo Zone of the Lachlan Fold Belt. We also obtained dates that confirm published ages for the Mount Bute Granite in the Stawell Zone, the Tynong pluton of the Tynong batholith in the Melbourne Zone and the Oberon pluton of the Wilsons Promontory batholith in the Bassian Zone. These dates confirm the Late Devonian (Fransian) age of most of this widespread plutonism, as well as the reported Emsian age of the Wilsons Promontory batholith. However, part of what has been mapped as the Mount Wombat pluton of the Strathbogie batholith could be significantly older than the rest.KEY POINTSMany rounded or embayed cores in zircon crystals are antecrysts rather than inherited detrital grains.The Mount Disappointment pluton is confirmed as Late Devonian in age, at 375.2 ± 2.5 Ma.At 378.2 ± 2.0 Ma, the Baringhup pluton of the Harcourt batholith is the same age as the Mount Alexander pluton, in the same batholith.Part of the Mount Wombat pluton in the Strathbogie batholith may be older than the rest of the pluton.
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Abstract The 15 major plutons comprising the 6000 km 2 Late Cretaceous Boulder batholith were emplaced at mid‐ to upper‐crustal levels (<20 km) during active thrusting in the Montana thrust belt. The Tobacco Root batholith, the largest satellite pluton of the Boulder batholith (< 300 km 2 ) was emplaced into uplifted Archean basement rocks immediately southeast of the frontal margin of the thrust belt. Emplacement of the Boulder batholith and its satellite plutons was controlled principally by three long‐lived and deeply penetrating regional fault sets. Two of these sets (a NE‐trending set and an E‐trending set) appear to have controlled the emplacement of the main mass of the batholith (Butte pluton). A third (NW‐trending) fault set controlled the emplacement of the Tobacco Root batholith and many of the smaller plutons of the Boulder batholith. The intersecting fault sets give the Butte pluton a distinctly rhomboid shape, elongate in a northeasterly direction. Geophysical data suggest that the pluton may have a flat floor at the level of the basal décollement of the thrust belt (∼17km). One mode of pluton emplacement consistent with a rhomboid pattern and with a compressional tectonic setting is the filling of the pull‐apart regions of shear zones. We propose that the main mass of the Boulder batholith was emplaced along a pull‐apart within a segment of a thin‐skinned thrust sheet during ENE translation in the thrust belt. Sinistral slip on the E‐trending faults at the north and south margins allowed the rhomboid‐shaped cavity to form within the thrust sheet above the basal décollement and to progressively fill with magma below a roof of cogenetic volcanic rocks. Late Cretaceous movement on the NW‐trending fault set was oblique with roughly equal components of sinistral slip and reverse slip. The main mass of the Tobacco Root batholith is elongate in a northwesterly direction between two of these faults and has a crudely rhomboid map pattern. We suggest that it, too, was emplaced by filling a sinistral pull‐apart between these faults.
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The ages of biotite and/or hornblende separates from 26 plutons in the eastern and southern Sierra Nevada were determined by the K-Ar method.Both biotite and hornblende age determinations were made for 14 of these samples.Six of the biotite-hornblende pairs give concordant ages; the rest of the biotites give younger ages.Two samples of hornblende give ages considered to be too young, which may be a consequence of post-magmatic potassium metasomatic alteration of hornblende.Most of the dated plutons were emplaced in Late Cretaceous time at the culmination of magmatic activity in the Sierra Nevada.Samples of two plutons not cut by the Independence dike swarm have ages of around 123 Ma; the dike swarm must have been emplaced prior to this time.
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