Structure and age of the Lower Magdalena Valley basin basement, northern Colombia: New reflection-seismic and U-Pb-Hf insights into the termination of the central andes against the Caribbean basin
Josué Alejandro Mora-BohórquezMauricio Ibáñez-MejíaOnno OnckenMario de FreitasVickye VélezAndrés MesaLina Serna
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Batholith
Basement
North American Plate
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Felsic
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Granitoid plutons are a major component of pre-Carboniferous rocks in Cape Breton Island and knowledge of the time and tectonic setting of their emplacement is crucial for understanding the geological history of the island, guiding exploration for granite-related economic mineralization, and making along-orogen correlations. The distribution of these plutons and their petrological characteristics have been used in the past for recognizing both Laurentian and peri-Gondwanan components in Cape Breton Island, and for subdividing the peri-Gondwanan components into Ganderian and Avalonian terranes. However, ages of many plutons were assumed on the basis of field relations and petrological features compared to those of the relatively few reliably dated plutons. Seventeen new U–Pb (zircon) ages from igneous units reported here provide enhanced understanding of the distribution of pluton ages. Arc-related plutons in the Aspy terrane with ages of ca. 490 to 475 Ma likely record the Penobscottian tectonomagmatic event recognized in the Exploits subzone of central Newfoundland and New Brunswick but not previously recognized in Cape Breton Island. Arc-related Devonian plutonic activity in the same terrane is more widespread, continuous, and protracted (445 Ma to 395 Ma) than previously known. Late Devonian magmatism in the Ganderian Aspy terrane is similar in age to that in the Avalonian Mira terrane (380 to 360 Ma) but the tectonic settings are different. In contrast, magmatic activity in the Bras d’Or terrane is almost exclusively arc-related in the Late Ediacaran (580 to 540 Ma) and rift-related in the Late Cambrian (520 to 490 Ma). The new data support the terrane distinctions previously documented.
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Rubidium and strontium concentrations and (87)Sr/(86)Sr values are documented herein for samples of plutons and associated rocks from 238 locations in the southern Sierra Nevada and vicinity. The goals of the investigation were to determine ages of rock units, to aid in the separation of plutons in poorly exposed areas, to determine the pattern of variation of initial (87)Sr/(86)Sr (hereafter called Sr(i)) for plutons, and to constrain more rigorously the boundaries of the continental Sierran and Salinian-western Mojave terranes defined on the basis of the Sr(i) of their plutons by Kistler (1978) and by Kistler and Peterman (1978). These new data expand the boundaries of the Salinian-western Mojave terrane and make it part of the Panthalassan lithosphere that lies west of the tectonic boundary, whereas the Sierran terrane is made part of the North American lithosphere that lies east of the tectonic boundary.
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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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Mylonite
Thrust fault
Butte
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Diorite
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Foliated intrusions of the Early Jurassic Aishihik Plutonic Suite (APS), including the Aishihik Batholith, have been included in Stikinia and interpreted as allochthonous with respect to adjacent terranes, including the Nisling and Yukon-Tanana Terranes. The Nisling Terrane was thought to lack Early Jurassic igneous rocks. However, the Aishihik Batholith, a single plutonic body that crystallized at ca. 187 Ma, forms a west-tapering lopolith or sheet-like body that intrudes deformed strata of the Nisling Terrane.
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Sill
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