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1.
Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K2O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses.Two foliated plutons in the southeast are 1,167±12m.y. (2) old, with distinctly different initial 87Sr/86Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial 87Sr/ 86Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial 87Sr/86Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial 87Sr/86Sr = 0.7061±0.0003. All of the intrusive rocks have initial 87Sr/86Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.  相似文献   

2.
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   

3.
Anorogenic granites of middle to late Proterozoic age in the Davis Inlet — Flowers Bay area of Labrador are subdivided on the basis of petrology and geochemistry into three coeval suites. Two of these are high-temperature anhydrous hypersolvus granites: a peralkaline aegirine-sodic-calcic to sodic amphibole-bearing suite and a non-alkaline fayalite-pyroxene-bearing suite. The third is a group of non-alkaline subsolvus hornblende-biotite-bearing granites. Associated with the hypersolvus peralkaline suite is a group of genetically related syenites and quartz syenites. The granites cut ca. 3,000 Ma old Archaean gneisses as well as Elsonian layered basic intrusions of the Nain Complex. One of these, a crudely layered mass which ranges in composition from gabbro to diorite and monzonite, appears to be related to the syenites. The peralkaline granites and some of the syenites are extremely enriched in the high field-strength elements such as Y, Zr, Nd, as well as Rb, Ga and Zn, and have low abundances of Ba, Sr and most of the transition elements. In contrast, the non-alkaline hypersolvus and subsolvus granites do not show the same degree of enrichment. Concentration of the highly charged cations in the peralkaline suite is believed to be the result of halogen-rich fluid activity during fractionation of the magma. The sodic evolution trend in the peralkaline suite is reflected mineralogically by the development of aegirine and aegirine-hedenbergite solid solutions, and by a spectacular amphibole compositional range from katophorite through winchite, richterite, riebeckite to arfvedsonite and ferro eckermannite. Accessory phases which are ubiquitous in these rocks include aenigmatite, astrophyllite, fluorite, monazite and zircon. The non-alkaline hypersolvus granites typically contain iron-rich phases such as fayalite, eulite, ferrosilite-hedenbergite, and annite rich biotite. In the subsolvus granites, amphiboles range in composition from edenite through common hornblende to actinolite and also coexist with annite-rich biotite.Whole-rock and mineral isotopic data for the different suites yield isochrons that are within error of ca. 1,260 Ma, but they have variable initial 87Sr/86Sr ratios. The initial 87Sr/86Sr of the syenites and peralkaline granites (0.7076±11) is significantly lower than the initial 87Sr/86Sr of the subsolvus granites (0.7138±22). These isotopic data provide further confirmation of the importance of a late Elsonian alkaline event in Labrador which can be correlated with Gardar igneous activity in south Greenland. The petrogenesis of the peralkaline suite is interpreted to reflect the effects of fractionation of anhydrous phases from mantle derived basic magma which was contaminated during ascent by radiogenic partial melts of crustal derivation. The non-alkaline hypersolvus and subsolvus granites are interpreted as crustal melts which formed under conditions of variable in response to the same thermal event, and which subsequently experienced feldspar fractionation during crystallization.  相似文献   

4.
Initial 87Sr/86Sr rations were determined for more than 80 plutonic rocks in Japan. The 87Sr/86Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.Initial 87Sr/86Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.Initial 87Sr/86Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial 87Sr/86Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.Initial 87Sr/86Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and 34S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial 87Sr/86Sr and 34S indicating interaction of the granitic magma with young sedimentary rocks enriched in 32S.  相似文献   

5.
Whole rock Rb–Sr measurements were made on associatedbasic and acid intrusives and extrusives from the northern andsouthern ends of the Lebombo monocline with a view to determineif the acid rocks came from the mantle or from the crust. Inthe south, in Swaziland (now called Ngwame) and Zululand, rhyolitesand granophyres (age 202 14 m.y.) have an initial Si87–Sr86ratio of 0.7042 0.0005; in the north, in the Nuanetsi syncline,rhyolites (age 206 13 m.y.) and granites from ring complexes(age 177 7 m.y.) have initial ratios of 0.7081 0.0008 and0.7085 0.0007, respectively. The initial ratios of the basicrocks, basalts and gabbros, vary in both areas and have a rangefrom 0.7042 to 0.7125 which can be explained by crustal contamination.The low initial ratios and isotopic homogeneity of the acidmagmas over long distances (160 km) in Swaziland and Zululandare interpreted as implying that they were derived from themantle.  相似文献   

6.
I.E.M. Smith  W. Compston 《Lithos》1982,15(3):199-206
Strontium isotope data are presented from 28 rock samples representing four distinct episodes of late Cenozoic volcanism in southeastern Papua. Eocene tholeiitic basalts have initial 87Sr/86Sr ratios (0.7037) which are higher than those in chemically comparable basalts and are thought to have been enhanced by rock-sea water interaction. Late Cenozoic are trench type volcanoes in the Papuan islands have initial 87Sr/86Sr ratios which show little variation (0.7041±2) in contrast to those from the contiguous Papua New Guinea mainland (0.7036-0.7054). This isotopic discontinuity does not appear to be due to contamination by immediately underlying sialic metamorphics. High-K trachytes in the Lusancay Islands north of the late Cenozoic are have comparable initial 87Sr/86Sr ratios. Wide variation (0.703-0.710) in the initial ratios measured in a group of apparently closely related rocks ranging in composition from transitional basalt to peralkaline rhyolite cannot be explained by differences in age or by late magmatic fractionation and continue to pose an enigma.  相似文献   

7.
G. Bylund  P.J. Patchett 《Lithos》1977,10(1):73-79
The stable palaeomagnetic pole of peralkaline syenites from the Särna intrusion lies close to the mean Upper Carboniferous pole for stable Europe and to other poles of similar age from the Baltic Shield. This age evidence is in agreement with an Rb-Sr 8-point mineral isochron for a single whole-rock sample of 287±14 m.y. (2σ). The isochron age and the pole position are believed to reflect the emplacement and cooling of the body and a comparison is drawn with other events of similar age in the Baltic Shield. Variation of initial 87Sr/86Sr ratios of whole-rock samples from 0.7041 to 0.7070 (at 287 m.y.) is considered to be caused by contamination of an ultimately mantle-derived magma by radiogenic strontium from crustal material.  相似文献   

8.
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   

9.
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   

10.
The Terra Nova ultrapotassic igneous rocks of northeastern Brazil consist of two dike swarms (alkali-feldspar syenites to quartz syenites and alkali-feldspar granites) and one elongated E-W syenitic body (the Serra do Livramento pluton), which intruded metasediments of the Cachoeirinha-Salgueiro fold belt from 580 to 514 Ma. Mafic ultrapotassic syenite enclaves are recorded in the Serra do Livramento and Terra Nova shoshonitic plutons, both of which are cut by the dike swarms.

Mineralogically, Terra Nova ultrapotassic hypabyssal rocks resemble shoshonitic lamprophyres. Pyroxene is present in all facies; the clinopyroxenes are zoned, SiO2 saturated, and Al2O3 poor (0.12 to 1.15%), and range from earlier diopside to late acmite. Amphiboles are characterized by high SiO2 and low Al2O3 (0.20 to 2.00%) and TiO2 (0.0 to 1.76%) contents; their compositions range from calcic to alkaline. The late amphiboles are riebeckite-arfvedsonites instead of K-richterites, as expected in ultrapotassic rocks, reflecting the early crystallization of K-feldspar.

The syenitic-facies rocks are mostly peralkaline, whereas the granites are metaluminous. The syenites have high concentrations of incompatible elements (Ba, Sr, and Rb) and light-rare-earth-element (LREE) concentrations lower than for typical ultrapotassic rocks, with chondrite-normalized Ce/Yb ratios of 10 to 20 and wide variation in the La/Ta ratios (40 to 250). The granites have lower incompatible-element contents and La/Ta ratios (20 to 60) than do the syenites.

Syenites from the dike swarm exhibit high initial 87Sr/86Sr ratios (0.7106), whereas εNd, values for the ultrapotassic mafic enclaves range from -1.1 to -3.7, suggesting that the enclaves and the syenites have different sources.

Field evidence, combined with geochemical data, shows that the granites and the syenites alternate in space and time, suggesting that syenites and granites cannot be associated by either fractional crystallization or partial melting of the same source. The syenites probably represent partial melting of a metasomatized lithospheric mantle, modified by subduction-zone fluids and crustal metasediments during the early stages of a Brasiliano (Pan-African) collisional event. Low-degree partial melting of a metasomatized lower crust appears to be the source of the granites.  相似文献   

11.
Concentrations of Rb, Sr, and REE (rare earth elements), and Sr-isotopic ratios in rocks of the Cambrian alkaline complexes in the Wet Mountains area, Colorado, show that rocks formed as end-products of a variety of magmas generated from different source materials. The complexes generally contain a bimodal suite of cumulus mafic-ultramafic rocks and younger leucocratic rocks that include nepheline syenite and hornblende-biotite syenite in the McClure Mountain Complex, nepheline syenite pegmatite in the Gem Park Complex, and quartz syenite in the complex at Democrat Creek. The nepheline syenite and hornblende-biotite syenite at McClure Mountain (535±5m.y.) are older than the syenitic rocks at Democrat Creek (511±8m.y.). REE concentrations indicate that the nepheline syenite at McClure Mountain cannot be derived from the hornblende-biotite syenite, which it intrudes, or from the associated mafic-ultramafic rocks. REE also indicate that mafic-ultramafic rocks at McClure Mountain have a source distinct from that of the mafic-ultramafic rocks at Democrat Creek.In the McClure Mountain Complex, initial87Sr/86Sr ratios for mafic-ultramafic rocks (0.7046±0.0002) are similar to those of hornblende-biotite syenite (0.7045±0.0002), suggesting a similar magmatic source, whereas ratios for carbonatites (0.7038±0.0002) are similar to those of nepheline syenite (0.7038±0.0002). At Democrat Creek, initial ratios of syenitic rocks (0.7032±0.0002) and mafic-ultramafic rocks (0.7028±0.0002) are different from those of corresponding rocks at McClure Mountain.  相似文献   

12.
Twenty-nine Rb-Sr whole-rock isotopic analyses and three U-Pb zircon analyses on foliated granites and largely unfoliated charnockitic rocks indicate that the central part of the Pan-African belt in west Africa was characterised by intense orogenic plutonism. These data and Rb-Sr analyses on muscovite books from late cross-cutting pegmatites indicate that the peak of magmatic activity occurred 610 ± 10 m.y. ago.Initial 87Sr/86Sr ratios for the granitic and charnockitic rocks are in the range 0.7065–0.7125, and indicate a significantly older crustal component in the magmas.  相似文献   

13.
The anorogenic igneous rocks of Mull consist essentially of a lava pile of predominantly basaltic composition, cut by an intrusive complex. The basement consists of Precambrian metamorphic rocks of the Moine Series underlain by Lewisian gneiss. The intrusive complex contains a significant proportion of granitic intrusions which can be ascribed to three successive centres of activity, Centres 1–3. We report new major and trace element, including rare earth element analyses, 87Sr/86Sr ratios and 18O values for a comprehensive collection of granitic rocks from the 3 centres. The 18O values range from +4 to –6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of 18O with other major and trace element data and 87Sr/86Sr ratios are uniformly low, apart from Fe2O3. It is thus unlikely that the interaction of the rocks with meteoric water has systematically altered the chemical (including Sr isotope) characteristics. The chemical and Sr isotope data reflect magmatic values and can therefore be used to comment on the petrogenesis of the granitic rocks.These data indicate that there are important differences between granitic rocks of the centres with Centre 1 forming one distinct group and Centres 2 and 3 a different group. For a given SiO2 value, the Centre 1 granites have higher Na2O, MgO, P2O5, TiO2, Sr, and V and lower Al2O3, MnO, Zn, Zr, and Y than those of Centres 2 and 3. In addition, the Centre 1 granites have lower REE contents and higher CeN/YbN ratios than those of Centres 2 and 3. Granites from all three centres have Eu anomalies, those of Centre 3 being generally greater (Eu/Eu* = 0.66–0.10). Finally, there are important Sr isotope distinctions between the three Centres; calculated initial 87Sr/86Sr ratios for the Centre 1 granites (using 58.2Ma) range between 0.71366–0.71646 (average 0.71530) and have a general correlation of 87Sr/86Sr with 87Rb/86Sr. The initial 87Sr/86Sr ratios of the Centre 2 granites range from 0.70663 to 0.70868, but the 87Sr/86Sr data do not define an isochron. Finally, data for the Centre 3 granites define a Rb-Sr wholerock isochron with an age of 58.2±2.5 Ma and an initial 87Sr/86Sr ratio of 0.71003 ±36.Both the chemical trends and isotopic data for the Mull granites can be interpreted in terms of contrasted origins for the granitic rocks of the two groups. The relatively primitive chemical composition and high initial 87Sr/86Sr ratios of the Centre 1 granites indicate a substantial crustal contribution and we consider that these granites formed by a combination of partial melting of Lewisian basement together with some magma derived by fractional crystallization of basaltic magma. In contrast, the chemical and isotope data for the Centre 2 and 3 granites are consistent with formation dominantly by fractional crystallization of basaltic magma, together with a relatively small proportion of crustal contamination. A model is proposed which emphasises that acid magmatism in Mull is a consequence of the rise and crystallization of basic magma into continental crust. Granite magma has formed both by partial melting and by fractional crystallization and both of these events probably occurred under open system conditions.With oxygen isotope analyses by J.J. Durham, Geochemical Division, Institute of Geological Sciences, 64–78 Grays Inn Road, London, WC1X 8NG, England  相似文献   

14.
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15.
Rb-Sr whole rock isochrons for the Monchique and Mt. Kenya complexes, together with a series of 87Sr/86Sr ratios for 7 other alkaline suites, reveal limited initial ratio variation within the oceanic basalt range. Such variation is ubiquitous in alkaline suites and renders interpretation of their Sr behaviour highly subjective; this has led to major interpretive inconsistencies between previous studies. A consideration of wall-rock reaction, responsible for a few anomalous ratios in the present data, is believed to reconcile many of these inconsistencies; the subjective element can also be minimised by using the total of available data as a basis for comparative interpretation instead of interpreting individual sets of data absolutely. This approach shows available data to be remarkably unified, implying less diversified petrogenesis than is apparent from the sum of interpretations for individual alkaline complexes. In particular, Sr data lend little support to various models invoking crustal participation in the origin of nephelinites, nepheline syenites, ijolites or peralkaline oversaturated rocks, but suggest that these generally are straight-forward derivatives of mantle liquids. The consanguinity of most alkaline complexes is also confirmed.  相似文献   

16.
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   

17.
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   

18.
The anorogenic complex of Evisa (Corsica) is made up of riebeckite hypersolvus granite and of albite-riebeckite-aegirine granite. Ten samples from the southern part of the complex provided Rb-Sr whole-rock isochrons for each facies. The ages of the two granites are indistinguishable at 246±7m.yr. corresponding to the Upper Permian. 87Sr/86Sr initial ratio (0.7034±0.0011) is in the mantle range of values and precludes any important crustal contamination. Li and Rb contents are controlled by the peralkaline fluid phase, reflected by deuteric changes. These alterations are weak in the hypersolvus facies but are obvious in the albite facies: replacement of preexisting perthite by albite, late precipitations of aegirine and fluorite, associated weak mineralization. The low value of 87Sr/86Sr initial ratio and the similarity of this ratio for both facies indicate that the fluid phase interacted with the crystallized rocks soon after the emplacement of the complex and provoked autometamorphic reactions without important external supplies.  相似文献   

19.
Initial 87Sr/86Sr ratios have been plotted against time of formation for various rock-types in the Schwarzwald and Vosges basement areas. These data exhibit strong positive correlations between the two variables. The Schwarzwald data define two distinct trend lines on the diagram, for the time span 500 to 250 Ma ago. The first comprises the data from orthogneisses, diatexites and pre-tectonic granites and documents the Sr isotope evolution in the crust underlying the Sehwarzwald. This region of the crust had a 87Rb/86Sr ratio of about 1. The other is delineated by data points from the post-tectonic plutons. These form a band corresponding to the development of closed systems with 87Rb/86Sr ratios of between 10 and 20 as indicated by the slope of the band. The second trend is interpreted as resulting from the formation of large, stable, deep-seated magma chambers formed by segregation of anatectic melts during a phase of tectonism 330Ma ago. The high 87Rb/86Sr ratios of these magmas arose by fractional crystallization, in response to tectonic decompression, during the coalescence of the presumedly water-saturated melts. Data for rocks from the Vosges, taken from the literature, form a single development line. Its slope corresponds to a 87Rb/86Sr ratio of about 5 and the line is analogous to the second trend line defined by the Schwarzwald data. The differences in the Rb/Sr systematics between the two areas can be explained as resulting from the deeper level of erosion in the Vosges. An important implication for Rb/Sr studies is that co-magmatic rocks can have very different initial 87Sr/86Sr ratios so that discrepancies in this ratio between rock-types cannot be used to divide plutons into genetic suites. On the other hand essential information is contained in the Rb/Sr systematics of orogenic magmatic rocks, such as the Variscan plutons in the Schwarzwald, that cannot be obtained from geochemical and petrological studies alone.  相似文献   

20.
Small volumes of peralkaline granites were generated as the final phase of a Pan African calc-alkaline igneous event which built the Arabian Peninsula. The peralkaline granites are closely associated with trends or sutures related to ophiolites. Peralkaline rocks are chemically heterogeneous, with anomalous abundances of Zr (average 2,150 ppm±2,600 1), Y (200±190), and Nb (105±100), representing up to ten-fold enrichments of these elements relative to abundances in calc alkaline granite counterparts. Large enrichments of some rare earth elements and fluorine are also present.The peralkaline granites have scattered whole rock 18O values, averaging 8.7±0.6% in the Hadb Aldyaheen Complex and 10.7±1% in the Jabal Sayid Complex. Quartz-albite fractionations of 0.5 to 1.5% signify that the heavier whole rock -values probably represent the oxygen isotope composition of the peralkaline magma. Small variable enrichments of 18O, in conjunction with slightly elevated 87Sr/86Sr initial ratios relative to broadly contemporaneous calc alkaline granites, are both suggestive of a small degree of involvement of crustal, or crustal derived material in the peralkaline magmas. It is proposed that the peculiar magma genesis is associated with a relaxation event which followed continental collision and underthrusting of salt rich sediments.  相似文献   

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