The Trawenagh Bay Granite and a new model for the emplacement of the Donegal Batholith
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ABSTRACT The Trawenagh Bay Granite (TBG) is shown to be a tabular pluton with gently inclined contacts that, from anisotropy of magnetic susceptibility (AMS) studies, was emplaced as a series of flow lobes whose geometries indicate that it flowed horizontally towards the W out of late stage adjacent steeply inclined monzogranite sheets of the Main Donegal Granite (MDG). We thus confirm in detail the central broad idea of the Pitcher & Read (1959) model that the Main Donegal Granite fed the Trawenagh Bay Granite. Early TBG flow lobes cut and are cut by deformation associated with the sinistral shear zone in which the MDG lies, thus demonstrating synchronicity of shearing and magmatism. The TBG magma leaked out of the shear zone and emplaced into undeformed country rocks and was probably guided by shear zone splays that die out along its northern and southern margins. At a late stage in the development of MDG, the splays developed from the NNE-trending SW boundary of the shear zone and caused a gap in this structure through which TBG magma was channelled out of the MDG. A review is presented of the last twenty-five years of published and unpublished work on the batholith, showing that the MDG shear zone was a long-lived structure almost certainly in existence before the emplacement of that body, and that four of the contiguous granitiods (Thorr, Ardara, and Rosses, as well as Trawenagh Bay) were all sourced within the shear zone. A new model is presented for the development of the batholith. The pre-existing crustal structure was a deep-seated N12°E fault in the basement to the Dalradian wall rocks of the granites, that was coupled to up to six other more minor WNW–ESE basement faults in the W. A NE–SW-trending sinistral shear zone was initiated at the end of the Caledonian orogeny, as calc-alkaline and deep-seated appinites were generated in the area. This shearing activated the pre-existing structures at the current crustal level, and the N12°E structure acted as a continental transform fault which allowed the dilation needed to facilitate the wedging space requirements of the MDG and the other units in the shear zone, as well as transferring regional sinistral shear through the system. The Thorr and Ardara plutons were emplaced first into the shear zone and then those magmas leaked out into the adjacent wall rocks: one to form a large laccolith, the other to form a balloon. Steep early MDG complex sheets (granodiorites and tonalities) were emplaced in the shear zone between the Thorr and Ardara emplacement sites. Dilation continued until late stage extensive monzogranite sheets were intruded in the NW and SE of the pluton. One of these probably leaked material westward to form the Rosses laccolith and southwestwards to form the TBG in the final stages of shear zone movement.Keywords:
Batholith
Transpression
Leucogranite
Lineation
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Transpression
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Transpression
Strain partitioning
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Transpression
Clockwise
Cleavage (geology)
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Felsic
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Lineation
Transpression
Mylonite
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Abstract The Heritage Range of the southern Ellsworth Mountains, West Antarctica, is composed of Cambrian to Permian sedimentary and volcanic rocks, which were deformed during the Permo-Triassic Gondwanian orogeny. The structural grain of the Heritage Range exhibits a previously unrecognized 18° swing from 333–153° in the north to 315–135° in the south. A change in structural and kinematic style accompanies this strike-swing, with a structure consistent with near-orthogonal shortening present in the south, and a structural style consistent with dextral transpression within the central and northern Heritage Range. Kinematic partitioning is present within the central Heritage Range, where strikeparallel, contemporaneous domains of dextral and reverse shear have developed simultaneously with the regional cleavage. Comparison of the structure and kinematics within both structural domains suggests that the central and northern Heritage Range experienced pure-shear dominated dextral transpression, with an approximate angle of relative shortening (α) of 65–70°. Results derived by integrating field data into a published kinematic partitioning model suggest relatively efficient kinematic partitioning has occurred. However, such efficient partitioning cannot be explained by strain partitioning models based purely on plate boundary conditions. Therefore, it is proposed that pre-existing weak structures were present within the unexposed basement facilitating the apparent high percentage of kinematic partitioning.
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The South Mountain Batholith is a peraluminous granitic complex ranging in composition from biotite granodiorite to muscovite-topaz ‘leucogranite’. Leucogranitic rocks (with generally <2% biotite) form a minor part (˜1⋅5%) of the batholith, and are of two types: (1) ‘associated leucogranites’ occurring as relatively small zones in fine-grained leucomonzogranites; and (2) ‘independent leucogranites’ forming generally larger bodies having no particular spatial association with other rock types. Mean chemical compositions of these two types of leucogranite are as follows (associated, independent): Na2O(3⋅46,3⋅83),K2O(4⋅40,4⋅09),and P2O5 (0⋅26, 0⋅45)in wt.%;Li(149, 281), F(1199, 2712), Rb (393, 725), U (7⋅4, 4⋅4), Nb (12⋅8, 23⋅4), Ta (2⋅9, 7⋅1), and Zr (52, 31) in ppm. Rare earth elements also differ between the two types (associated, independent): ΣREE (34⋅1 ppm, 19⋅9 ppm); and in the degree and variability of heavy REE fractionation (GdN/YbN=4⋅6±2⋅2, 2⋅0±0⋅7). In addition, associated leucogranite has REE compositions similar to those of its host rocks. Mean δ18O values (associated +ll⋅2±1⋅2‰, independent +ll⋅4±0⋅5‰; relative to SMOW) are comparable with the mean for the entire South Mountain Batholith (+l0⋅8±0⋅7‰). Radiometric dating (40Ar/39Ar on muscovite) shows that both types of leucogranite have identical ages of 372±3 Ma, equivalent to ages determined by other techniques for granodiorite and monzogranite samples elsewhere in the batholith. Field relations and geochemistry suggest that the associated leucogranite results from an open-system interaction between a fluid and its host leucomonzogranite, whereas the independent leucogranite bodies are discrete intrusions of highly fractionated melts that underwent closed-system, late-magmatic to post-magmatic fluid alteration. Where mineralized, the associated leucogranite characteristically hosts greisen-type or disseminated polymetallic mineralization, whereas the independent leucogranite hosts pegmatitic or disseminated polymetallic mineralization.
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Muscovite
Nova scotia
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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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Devonian
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Late Devonian extinction
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Diorite
Lithology
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