首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 906 毫秒
1.
A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 +27 ‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago.

All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial 87Sr/86Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in 87Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks).  相似文献   

2.
Rb‐Sr isotopic age measurements relate emplacement of the Pieman and Meredith Granites (356 ± 9 and 353 ± 7 m.y., respectively), and the Bischoff and Renison Bell Porphyries (349 ±4 and 355 ± 4 m.y., respectively) to the Tabberabberan Orogeny. The genetic relationship of the Bischoff Porphyry to mineralization and the agreement between the age of this porphyry and the age of the adjacent Meredith Granite, strongly suggests that the Bischoff mineralization resulted from granite intrusion.

The Pieman Granite is closely similar to the white Heemskirk Granite in displaying a high initial 87Sr/86Sr ratio (0.7354 ± 0.0018), feldspar discordance patterns and open system total‐rock behaviour. The high initial 87Sr/86Sr ratio is attributed to contamination during intrusion by Precambrian metasediments containing appreciable radiogenic strontium.

A basic intrusion (McIvors Hill Gabbro) gave a pre‐Tabberabberan age (518 ±133 m.y.) and a high initial 87Sr/86Sr ratio (0.7132 ± 0.0031).  相似文献   

3.
Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial 87Sr/86Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.  相似文献   

4.
The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial87Sr/86Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.  相似文献   

5.
Sixty-four Rb-Sr and two K-Ar isotopic measurements from seven ring complexes in central Nigeria provide evidence for a systematic age trend along a 200 km zone ranging from 174±5 m.y. in the north to 154±4 m.y. in the south. A peak of anorogenic magmatism occurred in the Jos Plateau region about 164±4 m.y. ago. Although a small syenitetrachyte complex at Zaranda, near Bauchi, gives an age of 190±15 m.y., unpublished ages of 290–330 m.y. for the southern Niger ring complexes confirm the existence of an overall southerly decreasing age trend in the Niger-Nigeria province of West Africa. Isotopic measurements on two small, oversaturated syenite intrusions at Zaranda and Pankshin suggest that syenitic liquids had initial 87Sr/86Sr ratios of 0.7048—not significantly different from the mantle range of values, but that related peralkaline silicic variants from the same complexes are depleted in total Sr and have higher 87Sr/86Sr initial ratios characteristic of the earth's crust. This variation of initial 87Sr/86Sr ratios in syenite-related granitic liquids of the peralkaline spectrum has also been noted at the Shere Hills, near Jos, and at Liruei, near Kano, and may be representative for all syenite-granite occurrences in the Nigerian Younger Granite province. Such isotopic variations in the initial 87Sr/86Sr ratio may be attributed to “crustal enrichment” of syenitic liquids whose source lies in the mantle. Coarse-grained, peraluminous biotite granites have consistently low initial 87Sr/86Sr ratios in the range 0.706–0.709 (similar to the ca. 600 m.y. Pan-African granites of the basement), and may represent further modifications of originally syenitic liquids in the crust, or the granites may have originated from an independent source within a “dioritio” lower crust. Although the magmatic trends show small variations in the initial 87Sr/86Sr ratio, much higher initial ratios are recorded in granites which have been modified within their roof zone by deuteric (autometamorphic) and/or metasomatic processes.  相似文献   

6.
The main rock types in the area north of the Frederikshåbs isblink are streaky gneisses, massive tonalites and ‘supracrustals’. The gneisses are thought to be the parent rocks of the tonalite and can be seen to merge into tonalite across a narrow zone of nebulite. Rb-Sr whole rock points from samples of gneiss and tonalite fall on a common isochron with an age of 2662 ± 116 m.y. (2σ) and initial ratio of 0.7032 ± 0.0008 (2σ) (half-life of 87Rb = 50 b.y.). The uncertainties in the isochron could mask small age and initial ratio differences between the gneiss and tonalite. However, our present interpretation is that the isochron reflects a homogenization of Sr isotopes within and between the two rock types. The presence of two out of four K-feldspar points on the whole rock isochron is interpreted as evidence that the K-feldspar became closed to Sr isotope migration at the same time as the whole rocks. Subsequent local isotopic disturbance has resulted in a minor loss of radiogenic strontium from two of the samples. The interpretation of the K-feldspar as a product of the epidoteamphibolite facies metamorphism allows the conclusion that the whole rock-K-feldspar isochron is recording a Sr isotopic homogenization during this event and is not related to the formation of the gneiss or the tonalite. Rb-Sr closure ages of ca. 2515 m.y. for muscovite and ca. 1950 m.y. for biotite could be recording separate isotopic disturbances or the cessation of strontium isotope migration as the minerals cooled through their characteristic blocking temperatures. Zircons from both the gneiss and the tonalite have igneous morphological features. Their U-Pb systems are complex, however, and suggest a multistage history of isotopic disturbance. Whereas the zircon U-Pb and whole rock Rb-Sr results suggest a maximum age of approximately 3000 m.y. for the parent rocks of the gneiss and tonalite they do not entirely exclude the possibility that the rocks represent older crust in which the isotopic systems have been almost completely reset ca. 2700 m.y. ago.  相似文献   

7.
The Nordre Strømfjord shear zone is about 20 km wide and is exposed for 170 km along strike. It formed under granulite to amphibolite facies conditions as part of the 1,850 Ma-old Nagssugtoqidian orogeny affecting the largely Archean terrain of central West Greenland. We have made Rb-Sr isotope measurements on ca. 130 whole rock samples from the shear zone and have made microprobe analyses of biotite and plagioclase on 40 of them in order to evaluate the effects of shear zone formation on the Rb-Sr isotope system. Seven separate localities were sampled, four of them extensively; large whole rock samples and small, cm-scale thin slices were used in this study.The Rb-Sr isotope systems and the biotite compositions of the large whole rocks show variable degrees of re-equilibration during shear zone formation. Rb-Sr data for the least affected suites still scatter about Archean isochrons and the biotite compositions show wide variations both within and between samples. In other more affected suites, the Rb-Sr system was reset to approximate the age of shear zone formation on the scale of 10's of meters, and the biotite compositions are consistent within individual samples. The most strongly affected suites underwent profound chemical and isotopic changes. Whole rock suites preserve virtually no Rb-Sr isotopic record of their preshear zone history and the biotite compositions of entire suites are very homogeneous. The cm-scale suites of thin slices all show some degree of Proterozoic re-equilibration, although sharp Sr isotopic discontinuities between groups of lithologic layers are preserved.Among the suites of large whole rocks, the observed variations do not correlate with variations in shear strain, size of sampling domain, average Rb/Sr, range of Rb/Sr or ppm Sr. The combined Rb-Sr and microprobe data suggest that the responses are most likely to represent differences in total fluid/rock ratio in a system undergoing significant fluid transport. Our data suggest an uneven distribution of fluid pathways within the shear zone and a strongly preferred direction of fluid flow parallel to lithologic boundaries.Data from previous studies are also consistent with the conclusion that fluid transport is the major agent of Rb-Sr isotopic resetting in shear zones. This conclusion has implications for optimum sampling strategies in Rb-Sr dating of high strain events and for the processes by which Rb-Sr isotopic resetting occurs during regional metamorphism.  相似文献   

8.
The Table Hill Volcanics of the Officer Basin were first dated as approximately 1100 m.y. from Rb‐Sr model ages for total‐rock samples of basalt from the Yowalga No. 2 bore. Later regional mapping, however, places the Volcanics as Marinoan (very late Precambrian) or younger, and receives support from discordant K‐Ar ages ranging from 330 m.y. to 445 m.y. New total‐rock analyses confirm the original Rb‐Sr data, but analyses of separated minerals do not confirm the low value for the initial 87Sr/86Sr that had been assumed to calculate the 1100 m.y. model age. Instead, apparently‐unaltered primary pyroxenes indicate that the initial 87Sr/86Sr could be as high as 0.718. Combined with the total‐rock results, this yields an apparent age for the basalt of 575 ± 40 m.y. It is possible in principle that the high 87Sr/86Sr in the pyroxenes could be due to Sr isotope exchange during a Palaeozoic metamorphism, but there is absolutely no field or petrological evidence for such an event. Consequently, and in view of the stratigraphic evidence for their age, the Rb‐Sr data are best interpreted as signifying an original extrusion of the basalts at 575 ± 40 m.y., together with a prehistory of the magma that includes contamination with radiogenic Sr and alkalis from Precambrian crustal material.  相似文献   

9.
Palaeomagnetic correlation in southern Africa predicts that the age of the Mashonaland dolerites of Rhodesia is confined within the limits of the age of the Waterberg System of South Africa, viz., between 1,950 m.y. and about 1,750 m.y. Rb-Sr data from the dolerites confirm this prediction. One sample gave a mineral isochron of 1,850 ± 20 m.y., which may be the true age of emplacement and is certainly a reliable minimum estimate for it. Total rock samples from nine dykes define an isochron of age 1,910 ± 280 m.y. and initial 87Sr/86Sr of 0.705 ? 0.002. In addition to variation in initial 87Sr/86Sr between dykes, there is also variation between minerals within single dykes presumeably due to contamination during crystallization and/or deuteric alteration.  相似文献   

10.
Rb-Sr whole rock data for gneisses and granites from the oriental Pyrénées yield cadomian ages for the basement gneisses and hercynian ages for the granites. Granites occur in two generations. An alkaline granite dated at 335±15 m.y. is syn.—or slightly post-tectonic, and a later calcalkaline granite dated at 275±12 m.y, is clearly intrusive and marks the end of hercynian tectono-metamorphism. Initial 87Sr/86Sr ratios of the granites put limits on the proportion of juvenile contribution from the mantle to their formation. Petrogenetic models are tested using Rb and Sr as tracers.  相似文献   

11.
Major element, Rb, Sr and Sr isotope abundances were examined in an ancient (Cambrian) weathering zone in the Butler Hill Granite, Missouri. Most of the major elements behaved predictably and systematically as a result of weathering intensity. MgO and the alkalis (Na2O and K2O) show highly erratic variations in both fresh and weathered rocks, suggesting considerable initial heterogeneity in the composition of biotite and feldspar. Rb predictably follows the behaviour of K during weathering (showing erratic variations similar to K), whereas Sr follows Ca (decreasing steadily as a result of weathering intensity). A method is postulated to date ancient weathering episodes using Rb-Sr data from a sequence of fresh to highly weathered samples. Data from the Butler Hill Granite (1185 m.y.) lend support to the proposed dating technique but show that more detailed sampling of the most intensely weathered regions is needed to produce more accurate results in future studies.  相似文献   

12.
The geochronology and genesis of the Qingyang batholith were investigated using40Ar/39Ar and Rb-Sr isotopic techniques. The Qingyang is a composite batholith consisting of two major rock types granodiorite and granite in the Yangtze fold belt.40Ar/39Ar spectra for biotite and amphibole separates are internally concordant. The concordance of the minerals and spectra indicate no thermal disturbance of the ages, and rapid cooling of the rocks. The granodiorite has an age of 137.6±1.4 m.y. and the granite 122.7±1.2 m.y. Whole-rock Rb-Sr analysis yields ages consistent with the40Ar/39Ar dates. Thus, the Qingyang batholith was formed in two major stages in the late Jurassic and early Cretaceous. The batholith is not Triassic as was previously proposed. Special40Ar/39Ar analysis of two granodiorite samples has precisely documented a 1.0 m.y. apparent age difference between these samples. Several factors could account for this difference, but different emplacement times seem most convincible. The granodiorite and granite show little variation in initial87Sr/86Sr ratio (about 0.7085). The high initial Sr ratios suggest that the magmas were formed by anatexis of older crustal materials.  相似文献   

13.
Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F 1 folding, whereas andalusite crystallized on a regional scale during the F 1 folding and in the post‐F 1 and pre‐F 2 static phase.

Rb‐Sr isotope data for total‐rock, feldspar, and muscovite samples of the meta‐sediment‐contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515 ± 8 m.y. and 506 ± 6 m.y. ago in the Late Cambrian epoch. Rb‐Sr and K‐Ar data for biotite from the granites record variable radiogenic Sr loss until about 469 m.y. ago and comparatively uniform radiogenic Ar loss until 460–475 m.y. ago. Rb‐Sr data for Kanmantoo Group metasediments and a metamorphic pegmatite indicate crystallization ages between 459–463 m.y. ago. Thus the regional andalusite‐grade temperatures and pressures, which appear responsible for the leakage of radiogenic Sr and Ar from biotite in the granites and the redistribution of Rb and Sr in the metasediments, seem to have persisted for some 50 m.y. after emplacement of the granites until the Early Ordovician epoch. There is evidence for further leakage of Sr and Ar from biotite in deformed granites from the margins of the intrusion more than 50 m.y. afterwards in the Late Silurian or Early Devonian, possibly during the F 2 folding.

Geological observations and radiometric data for other granitic rocks in southeastern South Australia, including the Palmer Granite, are consistent with this structural and metamorphic history of the Encounter Bay region.  相似文献   

14.
The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende 40Ar/39Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262.Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the 207Pb/206Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data.Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.  相似文献   

15.
The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene 40Ar-39Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite, disturbing both its Rb-Sr and Sm-Nd isotopic systems. The 87Sr/86Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source for more than 2% of the hydrothermal Sr. The initial 143Nd/144Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot, dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous limestone which then overlay the granite. Received: 3 August 1995 / Accepted: 11 April 1996  相似文献   

16.
The volcanics from Krivoklat-Rokycany complex (Barrandian synclinal of Bohemia) are investigated by the Rb-Sr whole-rock isochron method (λ 87Rb=1,47 10?11y?1). The age of 474±5 m. y. of these volcanics, stratigraphically overlain by the Tremadoc, implies that the Cambrian-Ordovician boundary, up to now estimated at 500 m. y. might be reset at about 470 m. y. The good quality of the isochron stresses the consanguinity of the different pétrographic terms of the Krivoklat-Rokycany volcanic series and the initial isotopic ratio87Sr/86Sr at 0,7041±0,0003 points out its infracrustal or mantle origin.  相似文献   

17.
Within the Pilbara Block of Western Australia, a complex of migmatite, gneissic and foliated granite near Marble Bar is intruded by a stock of younger massive granite (the Moolyella Granite) with which swarms of tin‐bearing pegmatites are associated. The age of the older granite has been determined by the Rb‐Sr method as 3,125 ± 366 m.y., and that of the Moolyella Granite as 2,670 ± 95 m.y. Initial Sr87/Sr86 ratios suggest that the older granite is close to primary crustal material, but that the Moolyella Granite consists of reworked material. It probably formed by partial remelting of the older granite.  相似文献   

18.
The Newer Volcanics Province of Victoria and South Australia consists of a major region of mainly alkaline basalts within which are two restricted areas containing strongly differentiated flow‐rocks. Typical alkalic basalts from this widespread province have K‐Ar ages from 4.5 to 0.5 m.y. and initial 87Sr/86Sr ratios from 0.7038 to 0.7045. Contrastingly, in the Macedon area of differentiated lavas, flow compositions range from limburgite to soda trachyte, with K‐Ar ages from 6.8 to 4.6 m.y. and initial 87Sr/86Sr ratios from 0.7052 to 0.7127. These differentiated rocks therefore are older, and some of them may have been contaminated by reaction with more radiogenic basement rocks during differentiation. Alternatively, the variation in initial Sr‐isotope composition may have resulted from varying isotopic composition of partial melts from the immediate source rocks. The most felsic of the differentiated rocks, soda trachyte, is extremely enriched with Rb relative to Sr; one of the three restricted outcrops of this rock (Camel's Hump) yields a total‐rock Rb‐Sr isochron age of 6.3 ± 0.6 m.y. with an initial 87Sr/86Sr ratio of 0.7127. K‐Ar sanidine ages reported for the three outcrops of trachyte are identical to each other and to the Rb‐Sr isochron result.  相似文献   

19.
Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial87Sr/86Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively. Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.  相似文献   

20.
The Charleston Granite from the Gawler Craton, South Australia, has been dated by the ion‐microprobe U‐Pb zircon method at 1585 ± 5 Ma (2σ). This confirms previous interpretations of population‐style U‐Pb zircon analyses which record a slightly older age due to the presence of inherited zircon. Inherited cores are present in many zircon crystals, and while the age of some cores can not be accurately determined due to extreme loss of radiogenic Pb, others have ages of ～ 1780, ～ 1970, and > 3150 Ma. These cores record a diverse crustal heritage for the Charleston Granite and indicate that ancient crustal material (> 3150 Ma) is present at depth in the Gawler Craton. This is also suggested by available Nd isotopic data for both the Charleston Granite and other Gawler Craton Archaean rocks. The Rb‐Sr and K‐Ar biotite ages from the Charleston Granite of 1560 to 1570 Ma are close to the U‐Pb zircon crystallization age and suggest that the granite has not experienced sustained thermal disturbance (> 250° C) since emplacement and cooling. However, a much younger Rb‐Sr total‐rock age of 1443 ± 26 Ma probably reflects low‐temperature disturbance to the Sr isotope system in feldspar.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号