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 31 毫秒
1.
In Tasmania shelly fossils are known from Middle and Upper Cambrian sediments of the Dundas Trough, Fossey Mountain Trough, Dial Range Trough, Beaconsfield Trough, Smithton Basin, Adamsfield Trough and from within sediments associated with the Mount Read Volcanics of Western Tasmania. In the Dundas Trough fossils range in age from early Middle Cambrian (Ptychagnostus gibbus Zone) to the middle Late Cambrian (pre‐Payntonian A or B). Late Middle Cambrian fossils occur in sediments associated with the Mount Read Volcanics in two places in Western Tasmania. Late Middle Cambrian fossils only are known from the Smithton Basin and the Beaconsfield Trough. Late Middle to early Late Cambrian faunas are known from the Dial Range Trough; the Adamsfield Trough contains middle Middle to middle Late Cambrian fossils. Tasmanian Cambrian faunas show affinities with those of Queensland, China, the northwest Siberian Platform and northern Victoria Land, Antarctica.  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.
Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and 207Pb/206Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The 87Rb/86Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. 87Rb/86Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The 87Rb/86Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton.  相似文献   

4.
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   

5.
A major west‐trending lineament marked by a wide belt of highly deformed rocks (the Redbank Zone), lies in the Arunta Complex, north of the Amadeus Basin. Along its southern margin the Zone has been progressively affected by, and is hence older than, migmatization and granite intrusion. The migmatization event yields a Rb‐Sr isochron age of 1076 ± 50 m.y. Within the migmatite complex, relicts of a pre‐migmatite metasedimentary sequence around the Chewings Range yield a Rb‐Sr isochron age of 1620 ± 70 m.y. The migmatites are unconformably overlain by the basal unit of the Amadeus Basin sequence, the Heavitree Quartzite. The 1076 ± 50 m.y. date thus provides a maximum age for the start of sedimentation along the northern margin of the Basin. The existence of a major zone of weakness in the basement probably exerted a strong control on basement and cover deformation during the Palaeozoic Alice Springs Orogeny.  相似文献   

6.
K-A ages from a variety of alkaline rocks, carbonatites (sövites) and alnöites from the Alnö complex range between 370 and 605 m.y. but the most meaningful ages concentrate between 546 and 578 m.y. Most nephelinites, phonolites and alkali trachytes that occur in a dike complex surrounding the main intrusion give a middle Cambrian RbSr whole-rock isochron age of 553±6 m.y. (2σ). Some samples (deleted from the age regression) were contaminated by radiogenic strontium. Plutonic pyroxenites, ijolites and nepheline syenites that form most of the core of the intrusion formed at about the same time and from the same parental magma as the dike rocks. Sövites and alnöites that crosscut the dike and core rocks did not intrude significantly later and may have formed from the same parent magma or mantle source. Many samples show evidence of either mixing or isotopic exchange of the magma with the country rocks during intrusion (fenitisation?) or open system behavior after crystallization. These processes resulted in some scatter of the data points about the best-fit RbSr isochron lines and in some anomalous K-A ages. Fenites have higher Rb/Sr and 87Sr/86Sr ratios than the alkaline rocks, making it unlikely that the latter were the remobilized products of extreme fenitization, as suggested by von Eckermann (1948). The Alnö intrusion is about the same age as the Fen complex of southern Norway, but is significantly older or younger than many other apparently similar intrusions in Scandinavia.  相似文献   

7.
A RbSr whole-rock isochron gives an age of 1168±21 m.y. for the agpaitic units of Ilímaussaq, showing that this complex belongs to the main phase of Gardar igneous activity in south Greenland and is not, as previously supposed, a significantly younger intrusion. Moreover, the agpaites must have intruded very soon after the earlier augite syenite phase of Ilímaussaq. The initial 87Sr/86Sr ratio of 0.7096±0.0022 for the agpaites is in marked contrast to the low (～0.703) ratio obtained for the augite syenites and suggests that selective enrichment of 87Sr occurred by preferential leaching of radiogenic strontium from unstable positions in Rb lattice sites in older crustal material by the highly reactive agpaitic magma.  相似文献   

8.
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   

9.
Some rocks of the Onverwacht Group, South Africa, have been analyzed for Rb and Sr concentrations and Sr isotopie composition. These rocks include volcanic rocks, layered ultramafic differentiates and cherty sediments. Whole rock data indicate that the Rb-Sr isotopie systems in many samples were open and yield no reasonable isochron relationships. However, the data of mineral separates from a basaltic komatiite define a good isochron of t = 3.50 ± 0.20 (2δ) b.y. with an initial Sr87/Sr86 ratio of 0.70048 ± 5(2δ). The orthodox interpretation of this age is the time of the low grade metamorphism. Since the basaltic komatiite is stratigraphically lower than the Middle Marker Horizon (dated as 3.36 ± 0.07 b.y. Hurley et al., 1972), and since it is commonly found that volcanism, sedimentary deposition, metamorphism and igneous intrusion in many Archean greenstone-granite terrain all took place in a relatively short time interval (less than 100 m.y.), it is reasonable to assume that the age of 3.50 b.y. might also represent the time of initial Onverwacht volcanism and deposition. The initial Sr87/Sr86 ratio obtained above is important to an understanding of the Sr isotopic composition of the Archean upper mantle. If the komatiite represents a large degree of partial melt (40–80 per cent) of the Archean upper mantle material, then the initial ratio obtained from the metamorphic komatiite should define an upper limit for the Sr isotopic composition of the upper mantle under the African crustal segment.  相似文献   

10.
Despite a wide latitude for interpretation of previous Rb–Sr isotopic data on the Oughterard Granite the age of this intrusion has been regarded as a critical time-marker in resolving the Caledonian evolution of Connemara. New isotopic data suggest that the age of the intrusion be revised from c. 460 Ma to c. 400 Ma, thus making the Oughterard Granite one among the many Newer Caledonian Granites in Ireland. The preferred age is 407 ± 23 Ma, and the initial 87 Sr/86Sr ratio is 0·7076 ± 1. Heterogeneity within the granite is demonstrated, which explains the difficulty in obtaining reliable isotopic ages from this intrusion.  相似文献   

11.
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   

12.
RbSr isotopic analyses of 10'whole-rock samples from the controversial peralkaline Norra Kärr complex of southern Sweden suggest an age (1580±62 m.y.) considerably older than had previously been anticipated, and indicate an initial 87Sr/86Sr ratio of 0.7072±0.0035 (errors at 2σ). The isotopic data are consistent with a primary magmatic origin for the Norra Kärr agpaites, but data from 8 mineral separates show that they have experienced at least one period of metamorphic disturbance since the original intrusion; the last episode of isotopic readjustment must have occurred after 1250 m.y. before present, and is attributed to the Sveconorwegian (Grenville) metamorphism.  相似文献   

13.
The U-Pb(Pb-Pb) age was determined for limestone from member III of the Sorna Formation out- cropped on the Azyr-Tal Range, Kuznetsk Alatau. The weighted average value from three calculated values is 523 ± 5 Ma (MSWD = 0.02, 2σ). This age coincides with the Early Cambrian age interval determined with Sr isotopic chemostratigraphy (87Sr/86Sr ratio is 0.70850–0.70852). The low value of μ2 = 238U/204Pb for limestone is due to the evolution of diagenetic fluid containing lead from the mantle reservoir into the Sorna sediments.  相似文献   

14.
The reported age of 1,100 ± 50 m.y. for the Albany Granite is taken as marking the waning stages of the main orogeny in the south coast of Western Australia. The high initial 87Sr/86Sr for this granite, 0.7118 ± 0.0064, suggests an anatectic or rheomorphic origin which is consistent with petrographic evidence for the absence of a later metamorphism. Shales from the Stirling Range Beds indicate a metamorphic event at 1,150 ± 40 m.y. which is considered to be the time of upthrusting of rocks of higher metamorphic grade to their present position adjacent to the Stirling Range Beds. The minimum age of deposition for the Stirling Range Beds is postulated as 1,340 m.y.  相似文献   

15.
Wulaga epithermal gold deposit is located in northeast China. Gold mineralization mainly occurs within the crypto‐explosive breccia belt of subvolcanic intrusion. Constraints on the precise timing of mineralization are of fundamental importance for understanding the ore genesis of the Wulaga gold deposit and its mineralization potential. Three hydrothermal stages have been identified: the early veiny quartz–euhedral pyrite stage; the fine pyrite–marcasite–gray or black chalcedony stage; and the late carbonate–pyrite stage. The Rb–Sr dating of gold‐bearing pyrites from the fine pyrite–marcasite–gray or black chalcedony stage is 113.8 ± 4.4 Ma with an initial 87Sr/86Sr ratio of 0.706346 ± 0.000019. The age of the gold deposit is consistent with the age of ore‐bearing volcanic (109–113 Ma) and subvolcanic intrusion (103–112 Ma) within the error limits, and the pyrite initial ratio has an identical value of 87Sr/86Sr to subvolcanic intrusion (0.705547 ± 0.000012). These indicate that crystallization of the wall rock and epithermal gold mineralization was coeval and likely cogenetic. Moreover, a lot of epithermal gold deposits that formed in Early Cretaceous volcanic and subvolcanic intrusions have been discovered in recent years in Heilongjiang province. Combined with the studies of tectonic and magmatic activities, we propose that the formation of the Wulaga gold deposit might be caused by the heated circum‐flow water related to the volcanic–subvolcanic intrusive hydrothermal event triggered by the ancient subduction of the Izanagi plate in the Early Cretaceous.  相似文献   

16.
A Rb-Sr whole-rock isochron study indicates that the entire Donegal granite suite was emplaced into orthotectonic Caledonian (Dalradian) rocks over a short interval during mid-Silurian to earliest-Devonian times. The Thorr pluton, probably the earliest member of the suite, yields an age of 418 ± 26 Myr and initial 87Sr/86Sr ratio of 0·7055 ± 4, while the latest member, the Main Donegal pluton has an age of 407 ± 23 Myr and initial 87Sr/86Sr ratio of 0·7063 ± 5 (Λ87Rb = 1·42 ± 10−11 yr−1). Errors on both the age and initial Sr isotope ratios incorporate both a priori and geological scatter components and are quoted at the 2-sigma level. The low and restricted range of initial Sr isotope ratios suggests small but significant differences in the composition of the parental granitic magmas which were derived from a low Rb/Sr, low 87Sr/86Sr source.  相似文献   

17.
Nineteen samples of granites, orthogneisses and paragneisses from the High Himalaya basement nappe(s) of the Mount Everest region have been dated by the Rb/Sr method. The post-metamorphic tourmaline leucogranites of the upper Imja Drangka (Nuptse, Lhotse Glacier) have high initial Sr87/Sr86 characteristic of an anatectic origin from crustal material. A whole-rock isochron age of 52 m. y. (Early Eocene) has been obtained for the samples from the granite body of Lhotse Glacier; apparently Sr isotopic homogenization was not reached throughout the much larger Nuptse granite. The granite precursor of the migmatitic orthogneisses from the upper Dudh Kosi valley has an age of 550 ± 16 m. y. (whole rock isochron) and a high initial Sr87/Sr86 ratio indicating its origin from an older basement complex. The Rb/Sr data on paragneisses from the south face of Lhotse do not define an isochron, possibly reflecting isotopic hetero-geneity in the sedimentary protoliths and incomplete homogenization during a late Precambrian metamorphism. All the mineral ages fall in the time span from 15 to 17 m. y. They represent cooling ages reflecting a regional phase of major uplift in the Middle Miocene and post-dating the peak of the Himalayan metamorphism which the data from the Mt. Everest region place in pre-Eocene times.  相似文献   

18.
Rb‐Sr and K‐Ar measurements have been made on five glauconite samples from the near basal Treuer Member of the Vaughan Springs Quartzite of the Ngalia Basin, Northern Territory, Australia. Comparison of results between and within the two groups of data demonstrates that variable losses of radiogenic strontium and argon have occurred, but allows a minimum age of 1280 m.y. to be calculated for the member. Sedimentation began in the Ngalia Basin shortly before the time of deposition of this member.

Regional correlations suggest that this minimum age applies to the adjacent Amadeus Basin as well.

Measurements were also made on glauconite from a single sample of the Lower Palaeozoic Djagamara Formation which is in the same sequence. It yields a mid‐Ordovician K‐Ar age which generally agrees with the broad range of post‐Lower Cambrian to pre‐Carboniferous age determined from fossil evidence in bounding formations. A low Rb/Sr ratio prevented calculation of a Rb‐Sr age, but the combination of K‐Ar age and Rb‐Sr measurements allowed an accurate initial 87Sr/86Sr ratio of .739 to be determined. This is much greater than ocean water values, and it appears that such information on young samples and/or those of low Rb/Sr ratio could help define the source material for glauconite formation.  相似文献   

19.
The Rb–Sr and 147Sm–143Nd age data obtained for sheeted dolerite dykes and rocks of the Platinum Belt of the Urals within the Tagil segment of the paleoceanic spreading structure (Middle Urals) are discussed. The study of the Rb–Sr isotope systematics of gabbro allowed us to reveal errochronous dependencies, which yielded ages of 415 and 345 Ma at (87Sr/86Sr)0 = 0.70385 ± 0.00068 and 0.7029 ± 0.0010, correspondingly. The 147Sm–143Nd isotope age data demonstrate a specific coincidence of the chronometric ages of the sheeted dolerite dyke complex (426 ± 54, 426 ± 34, and 424 ± 19 Ma) and gabbro from the Revda gabbro–ultramafic massif (431 ± 27 Ma) and from screens between dolerite dykes in the sheeted dyke complex (427 ± 32 Ma, 429 ± 26 Ma). The proximity of the 147Sm–143Nd ages of gabbro and dolerite can be explained by the thermal effect of the basaltic melt, which is the protolith for the dyke complex, on the hosting gabbro.  相似文献   

20.
The geochemistry and evolution of early precambrian mantle   总被引：1，自引：0，他引：1  
Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and 87Sr/86Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:
  1. starting with primordial Sr, a short period of relatively rapid 87Sr/86Sr growth, followed by Rb depletion;
  2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
  3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with 87Sr/86Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.
  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