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21.
Vincent J. Morand 《Australian Journal of Earth Sciences》2013,60(1):119-133
The Late Ordovician Abercrombie Beds, south of Reids Flat, New South Wales, and adjacent to the Wyangala Batholith, show evidence of three successive fold episodes. First generation folds are tight to isoclinal, with fold axes ranging from vertical to horizontal and north‐trending, and steep axial‐plane slaty cleavage. Second generation folds are steeply plunging, tight to open with north‐striking axial planes. In pelitic rocks the axial plane structure is a crenulation cleavage which overprints the slaty cleavage. The first two fold episodes were accompanied by greenschist‐facies metamorphism. Granite emplacement occurred prior to the second fold episode. A third deformation was of relatively mild intensity and produced open, north‐trending folds with axial planes dipping moderately to the east, and crenulation cleavage as the axial plane structure in pelitic rocks. These latest folds are correlated with the latest folds in the Abercrombie Beds north of the Abercrombie River. The mapped area has no apparent macroscopic structure and may be considered as a single domain. 相似文献
22.
Bryan E. Chenhall Brian G. Jones Paul F. Carr 《Australian Journal of Earth Sciences》2013,60(3):389-401
Contact metamorphism has been recognized along a 4 km wide belt adjacent to the shallow‐dipping eastern margin of the Arthursleigh Tonalite, an Early Devonian pluton of the Marulan Batholith, eastern New South Wales. In Ordovician psammitic and pelitic rocks three zones of progressive contact metamorphism range from muscovite + biotite + chlorite assemblages in the outer zone to K‐feldspar + cordierite assemblages adjacent to the pluton and in metasedimentary xenoliths. Retrograde phenomena include extensive replacement of metamorphic minerals by ‘sericite’ and chlorite. Calcareous metasediments adjacent to the tonalite typically contain assemblages of quartz + calcic plagioclase + ferrosalite + sphene, or wollastonite + calcite + diopside with minor grossularite and vesuvianite. Thermal effects in volcanic rocks along the western margin of the pluton are confined to recrystallization of the groundmass. The regional geology indicates confining pressures of approximately 1 kbar at the time of emplacement of the tonalite. Contact metamorphic temperatures were estimated from two‐feldspar geothermometry to attain a maximum of approximately 590°C for rocks in the innermost zone of the aureole and 700°C for the xenoliths. Fluid compositions attending progressive contact metamorphism were water‐rich (Xco2<0.2) and, during cooling, these fluids probably account for the extensive retrograde hydration observed in the aureole. 相似文献
23.
Geophysical data are presented that characterise a blind pluton, the Mountain Home Pluton (MHP), which intrudes the southern portion of the Bundarra Suite (BS), 30 km northeast of Bendemeer, New South Wales. A positive magnetic anomaly within the non-magnetic granites of the BS (Banalasta and Pringles Monzogranites) was previously identified as a sub-surface intrusion. Interpretation of new gravity data and analysis of aeromagnetic data are used to infer the depth, size, density, magnetic susceptibility and likely petrology of the pluton. The best-fit model indicates that the MHP is very similar to the Looanga Monzogranite, a felsic member of the Moonbi Suite of the New England Batholith (NEB) that intrudes the BS 5–7 km southeast of the MHP. The top of the MHP is inferred to lie about 1 km beneath the surface and the pluton extends to a depth of at least 6 km. Our model furthermore suggests that the southwestern margin of the MHP is subvertical, whereas a shallower dip (<45°) towards the north is proposed for the northeastern surface of the pluton. A north-trending dyke swarm, identified on the basis of linear positive magnetic anomalies, may be related to the MHP. This swarm of more than 20 relatively magnetic dykes extends out to about 10 km north from the pluton. Magnetic modelling of the dykes indicates that susceptibility values of the dykes are probably very similar to the range of the MHP, and also suggests the width of individual dykes (also not known to be exposed at the surface) to be at most a few tens of metres. A petrographic examination of the intruded BS granites at the surface suggests that metamorphic zoning as seen in mineralogical characteristics may be related to the underlying pluton. 相似文献
24.
25.
The Coastal Batholith of Peru extends over 1600 km parallel to the coast along the Andean trend. Gravity profiles on three traverses across the batholith indicate the geometry is essentially that of a flat slab with average thickness from 2.0–3.2 km, and a thick root 4–10 km wide to the west. Granitic material does not extend to depths greater than 3 km below sea level datum.This study supports recent gravity work which indicates plutons are commonly thin, 5 km or less in thickness. Detailed mapping in the Lima segment of the Coastal Batholith reveals thin plutons where space was made dominantly by downward displacement via floor depression. However, early roof uplift also created some space. Stoping occurs but is not a major space maker. Floor depression may be modelled by cantilever or piston mechanisms and although the strong marginal deformation with mylonites, tuffisites, microbreccia, faults and shear zones suggests the piston model best describes the mechanism of emplacement of much of the Coastal Batholith some space was probably made by a cantilever mechanism. In brief, space making processes involved early roof uplift and regional doming, then floor depression mainly by piston and probably subsidiary cantilever mechanisms and, finally, local stoping producing the cut-out rectilinear nature of the batholith.The Coastal Batholith formed on shallow partial melting of hydrous basaltic marginal basin rocks between 5 and 10 km. Floor depression occurred as the crustal column foundered into an actively deflating layer of partial melt. This is an efficient space making process and is limited here to shallow levels of the upper crust only. The melts ascended to within 2 or 3 km of the surface, up dyke-like conduits then spread horizontally to form tabular plutons. 相似文献
26.
Long-term thermal consequences of the redistribution of heat-producing elements associated with large-scale granitic complexes 总被引：3，自引：0，他引：3
Crustal thermal regimes are sensitive to both the amount and distribution of heat producing elements (HPEs). Since a significant proportion of the crustal complement of HPEs is contained within granites, granite generation and emplacement should lead to significant long‐term changes in the thermal structure of the crust. Using HPE concentrations appropriate to representative Australian Proterozoic granites we show that granite segregation leads to changes in the temperature field of the crust of up to c. 50 °C, producing long‐term cooling in the source regions and heating at emplacement levels, relative to the pre‐granite conductive thermal regime. Because of the intimate connection between thermal regime and lithospheric strength, granite‐assisted redistribution of HPEs is likely to be fundamental to cratonisation. 相似文献
27.
SHRIMP U–Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianópolis Batholith, southern Brazil. The results, supported by SEM backscattered and cathodoluminescence imagery, identify inherited zircon populations and confirm the long-lived, crustal recycling processes responsible for the accretion of the batholith. The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations, and enables discrimination to be made between inherited cores and melt-precipitated overgrowths. New dating of two post-tectonic plutons (samples 1 and 2) revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths. The U/Pb isotopic results from both samples spread along concordia by ca. 40 m.y. (sample 1) to 100 m.y. (sample 2), clustering in two closely spaced (bimodal), partially overlapping peaks. Melt-precipitated rims and homogeneous new grains, dated at ca. 600 Ma, furnish the crystallisation age of the plutons. The magmatic textured cores and xenocrysts dated at ca. 630–620 Ma are interpreted as inherited restitic material from supposedly short-lived (meta)granitic sources. The reassessment of previous SHRIMP data of two banded anatectic granitoids (samples 3 and 4) revealed more complex morphological patterns, in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims, dated at ca. 600 Ma and 592±2 Ma, respectively. Major populations of magmatic-textured inherited cores dated at 2006±3 Ma and 2175±13 Ma characterise samples 3 and 4, respectively. The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages (ca. 2900–620 Ma), suggesting partial melting of metasedimentary components. The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent (meta)tonalitic gneiss and amphibolitic country-rock (paleosome). The recognition of the (melt-precipitated) Neoproterozoic overgrowths and new crystals, and the restite provenance of the cores, supplants a previous interpretation of Paleoproterozoic magmatism (cores) and Neoproterozoic (solid-state) metamorphic overprint. As a major consequence of the former interpretation, the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianópolis Batholith/arc. 相似文献
28.
This paper investigates the reasons for the presence of both domed inselbergs (bornhardts) and boulder inselbergs (koppies) on the Matopos Batholith, Zimbabwe. XRF and microprobe analyses of rock samples from 16 bornhardts and 24 koppies have shown no significant chemical or mineralogical differences which might indicate greater weathering resistance of the bornhardts. Many bornhardts are located on massive bodies of slowly-cooled porphyritic granite which were emplaced at a relatively late stage in the evolution of the batholith. The surrounding rocks are less porphyritic and appear to have been foliated during deformation associated with emplacement of the later bodies. The entire batholith has been affected by post-emplacement tectonism which has created three major fracture systems. The morphological differences between bornhardts and koppies in this area are therefore considered to reflect differences in jointing which arise both from the primary igneous emplacement process and from tectonic deformation. 相似文献
29.
Abstract We report new field, petrological and isotopic data and interpretations from one of New Zealand’s major basement geological boundaries, the contact between the east side of the Median Batholith (formerly Median Tectonic Zone) and the allochthonous Mesozoic terranes of the Eastern Province. In the Nelson and Hollyford–Eglinton areas this contact is a Cenozoic fault, the Median Tectonic Line of earlier workers. However, in the Longwood Range, unfaulted pre-Cenozoic geological relations are preserved intact. Our new Ar–Ar, U–Pb and isotopic data show that the Median Batholith in the Longwood Range consists of two suites. (i) Eastern, isotopically primitive (87Sr/86Sri = 0.702 to 0.703; ?NdT = + 7 to + 8) trondhjemite and gabbroic rocks of Permian age that we believe are part of the intraoceanic Brook Street arc of the Eastern Province. (ii) Western, isotopically more evolved (87Sr/86Sri = 0.703 to 0.704; ?NdT = + 3 to + 5) quartz diorites, quartz monzodiorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Batholith. Field observations in the Longwood Range indicate intrusive, not faulted, contacts between these units and constrain accretion of the Brook Street Terrane to Gondwana to have occurred 230–245 Ma. Intra-batholith shear zones (T ~ 600°C and P ~ 0.2–0.3 GPa) were active at approximately 220 Ma. Modelling of K-feldspar Ar incremental heating ages indicate that most of the Longwood Range had cooled below 175°C by the Middle Jurassic and experienced no subsequent reheating. Significant additional post-accretionary Early Cretaceous and Cenozoic thermotectonic activity in Median Batholith in the Hollyford-Eglinton area is indicated by a new 140 ± 2 Ma U-Pb zircon date on a Largs ignimbrite, as well as by Cenozoic K-feldspar Ar–Ar ages in the Middle Triassic Mistake Diorite. 相似文献
30.
Seismic velocity structure of a large mafic intrusion in the crust of central Denmark from project ESTRID 总被引：1，自引：1，他引：0
The origin of regional sedimentary basins is being investigated by the ESTRID project (Explosion Seismic Transects around a Rift In Denmark). This project investigates the mechanisms of the formation of wide, regional basins and their interrelation to previous rifting processes in the Danish–Norwegian Basin in the North Sea region. In May 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central Denmark. The data confirms the presence of a body with high seismic velocity (> 6.5 km/s) extending from a depth of 10–12 km depth into the lower crust. There is a remarkable Moho relief between 27 and 34 km depth along this new along-strike profile as based on ray-tracing modelling of PmP reflections. The lack of PmP reflections at a zone of very high velocity in the lowest crust (7.3–7.5 km/s) suggests a possible location of a feeder channel to the batholith. The presence of volcanic rocks of Carboniferous–Permian age above the intrusion (mafic batholith) suggests a similar age of the intrusion. An older obliquely crossing profile and two new fan profiles deployed perpendicular to the main ESTRID profile, show that the batholith is about 30–40 km wide. The existence of this large mafic batholith supports the hypothesis that the origin of the Danish–Norwegian Basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late Carboniferous and early Permian. The data and interpretations from project ESTRID will form the basis for subsidence modelling. Tentatively, we interpret the formation of the Danish–Norwegian Basin as a thermal subsidence basin, which developed after widespread rifting of the region. 相似文献