تعیین سن بیوتیت–کل سنگ بروش Rb/Sr روی گرانیتوئید کاشمر

دسته پترولوژی
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری بیست و پنجمین گردهمائی علوم زمین
نویسنده ابوالفضل سلطانی
تاريخ برگزاری ۰۱ اسفند ۱۳۸۵

 

 Introduction

The Kashmar granitoid (۳۵o۱۵′–۳۵o۲۵′ N and ۵۸o۱۵′–۵۸o۵۵′ E) is the largest subvolcanic mass (~۲۰۰ km۲) occurring throughout the northern parts of the Kashmar area (Fig. ۱). It is bordered by the Doruneh Fault to the south and the Rivash Fault to the north, the faults defining the boundaries of Taknar Zone. The Kashmar granitoid intrudes into genetically related andesitic lavas and pyroclastic rocks (mainly tuff) of Early Eocene age [۱]. The granitoid together with associated volcanic rocks constitute the central part of the ‘North Doruneh Fault Magmatic Belt’ that runs from the northern to eastern margins of the Central Iran Plate (CIP) for a length of ~۳۵۰ km with a width of ۱۵ to ۸۰ km [۲]. In this belt, volcanic and plutonic rocks are associated with iron–oxide deposits [۳] and belong to Tertiary ages. They occur in an arc with a convexity to the north, defining the margins of the CIP. The Kashmar granitoid is partly overlain in the south by Neogene sandstone with ۱۰۰–۱۵۰ m thick. Where observed, contact of the granitoid with the surrounding rocks is either faulted (steeply deepened) or narrow low–grade hornfelsic rims which have developed in the Eocene volcanic rocks. Contacts between different plutons together with a longitudinal shape of the igneous assemblage are consistent with the east–west trend of the Doruneh Fault to the south. This implies that the emplacement of the Kashmar granitoid is most likely related to the activity of Doruneh Fault. According to modal petrography, homogeneous plutons include tonalite, granodiorite, granite and alkali feldspar granite. Among these, granodiorite and granite constitute about ۹۰% of the granitoid exposure. All the mentioned rocks are medium–grained, felsic in composition and contain low–temperature metaluminous minerals that can be classified as I–type which tend to occur along the continental margins [۴]. Biotite and magnesio–hornblende present as the only mafic silicates which coexist with zircon grains [۵]. However, zircon grains present in rocks with different SiO۲ contents. This indicates that the granite magma was saturated in zircon throughout its evolution, a feature attributed for low-temperature I-type granites. The homogeneous interlocking quartz–feldspar textures and the presences of aphyritic volcanic rocks at the same structural level indicate that these plutons were emplaced at very high levels in the crust and solidified simultaneously [cf. ۶]. Before this study, no isotopic or geochemical data have been reported for the Kashmar granitoid.

 

Sample Preparation and Analytical Techniques

For each sample ~۰.۱ mg of purified biotite was dissolved in teflon capsule, mixed with a Rb/Sr spike, by using a mixture of HF–HNO۳ acids. Analytical processes followed by isotope dilution method. Rb and Sr were separated and concentrated for mass spectrometric analyses using standard ion–exchange procedures. Also, representative subset of ۸ whole rock powders (۰.۱ mg) were analyzed for Rb, Sr and Sm, Nd isotope ratios, at School of Geosciences, University of Wollongong, and in CSIRO, Sydney, Australia.

Isotopic analyses were undertaken on a VG ۳۵۴ mass spectrometer in CSIRO, Sydney. Replicate analyses of SRM ۹۸۷ gave ۸۶Sr/۸۸Sr = ۰.۷۱۰۲۵۱±۲۸ (external precision at ۲s, n = ۱۷) and the JM–Nd standard gave ۱۴۶Nd/۱۴۴Nd = ۰.۵۱۱۱۱۱±۱۲ (external precision at ۲s, n = ۱۷). ۸۷Sr/۸۶Sr normalized to ۸۶Sr/۸۸Sr = ۰.۱۱۹۴, ۲s analytical uncertainty for ۸۷Sr۸۶Sr is ±~۰.۰۰۰۰۵. ۱۴۳Nd/۱۴۴Nd normalized to ۱۴۶Nd/۱۴۴Nd = ۰.۷۲۱۹, ۲s analytical uncertainty for eNd is ±~۰.۵ units. Ages, ۸۷Rb/۸۶Sr, ۱۴۷Sm/۱۴۴Nd, eNd and initial ratios (۸۷Sr/۸۶Sr and ۱۴۳Nd/۱۴۴Nd), were calculated by the CIS (Center for Isotope Studies) programs, and λ = ۱.۴۲ × ۱۰–۱۱a–۱. The MSWD, ۲s uncertainty in age and initial ۸۷Sr/۸۶Sr were calculated by standard percents of ۰.۸ and ۰.۰۰۸, respectively for X (۸۷Rb/۸۶Sr) and Y (۸۷Sr/۸۶Sr) axes.

 

 

Figure ۱. Geological map of the Kashmar granitoid, northeastern Central Iran۱:place> Plate.

Zirconium Variations in Lowand Hightemperature Granite Suites

Many granite suites particularly those of Lachlan Fold Belt, eastern Australia۱:place>۱:country-region> exhibit two contrasting patterns of Zr variation:

۱) Linear variation against SiO۲ contents, similar to the pattern of other elements for those suites. This is by far the more common type of behavior. It applies to all S-type suites, where the variation is more scattered, and to most I-type suites, where in most cases, the Zr content falls with increasing SiO۲, although sometimes it is fairly constant, and in a few cases it rises [۵]. In this case, the granite magma was saturated in zircon throughout its evolution. This has been confirmed by the presence of older zircon cores as restite (unmelted material) in most low-temperature granite suites. In the case of Kashmar granitoid, Zr-saturated feature is supported by the presence of zircon crystals in rocks with different silica contents.

۲) Inflected trends against SiO۲ contents, where the Zr rises from low values to a maximum at a little below ۷۰ wt% SiO۲, then falls back to low values. In this case, the granite magma was clearly not saturated in zircon during the first part of its evolution, but became saturated near the point of Zr inflection. Such a situation has been found in high-temperature granite suites in which magmas mainly evolved through the fractional crystallization.

 

Evidence for I–type Low–Temperature and Restite Fractionation

The Kashmar granitoid formed in low-temperature, low-pressure and oxidized condition from felsic I-type source rocks of lower continental crust, for the following reasons:

۱–The presence of metaluminous minerals and absence of Al-rich minerals (e.g. muscovite, garnet, etc.) in all rock types, indicating I-type protolith which is supported by high Na۲O contents (>۳.۵ wt%) and low K۲O/Na۲O ratio,

۲–The presence of abundant Fe–Ti oxide, titanite and absence or scarcity of ilmenite, suggesting magnetite-series granite formed in oxidized condition which is supported by the presence of pale pink K-feldspar and is consistent with the emplacement of the Kashmar granitoid in an iron-oxide type belt [۳],

۳–The average SiO۲ contents is ۶۶.۸ wt% which is consistent with dominant exposure of granodiorite. Accordingly, the Kashmar granitoid is I-(granodioritic) type of Caledonian-like Fold Belt,

۴–Zircon grains coexist with amphibole or biotite in different rock silica variants, indicating the Kashmar magmas were Zr–saturated and low in temperature (≤۸۰۰ oC), while, in high–temperature magmas (>۱۰۰۰ oC) zircon is not initially present in the melt because the melt is under–saturated in zircon,

۵–There are similarities in morphology and size between zircon grains from rocks with different silica contents and those hosted in microgranular enclaves, indicating zircon was not dissolved in the melt, hence zircon is restite in low-temperature condition,

۶-The REE show enrichment with steep LREE, moderate to strongly negative anomalies for Eu and flat HREE, indicating fractionation processes  and garnet-free residuum in the source, all characteristics of I-type magmas,

۷- The contents of compatible elements (e.g. Ni, Cr, V, Sc) are very low indicating low-temperature granites are products of magmatic recycling of older crust,

۸–The linear variation and negative trend of Zr–SiO۲ with the absence of inflexion support Zr–saturated low–temperature I-type melt, while in high–temperature granites (>۱۰۰۰ oC) as zircon starts to crystallize, Zr shows inflected trend against SiO۲ wt%,

۹–The absence of significant thermal aureole around the plutons of Kashmar granitoid indicate crystallization from a low temperature melt,

۱۰–The presence of microgranular enclaves (unmelted materials) with similar mineralogy as the host rocks, particularly in tonalities which contain the lowest SiO۲ content, suggests restite separated from magmas that were not completely molten,

۱۱–The quartzofeldspathic nature of Kashmar granitoid particularly the alkali feldspar granite is similar to haplogranites which formed from minimum-melt composition,

۱۲–The presence of microcline as a typical low–temperature K–feldspar which displays crossed-hatched (tartan) twinning, and presence of abundant microperthitic intergrowths indicate extreme evolution of felsic I-type granite magmas in a hypersolvous condition,

۱۳– The absence of cumulates such as high Ca, Mg, Cr and low K, Rb, Zr to indicate partial melting of mafic source rocks at largely or completely molten state due to high temperature,

۱۴–The content of P۲O۵ is low (<۰.۳۴ wt%) and decreases to a negligible amount towards higher silica variant, indicating low solubility of P in low–temperature I–type melts,

۱۵–The apatite crystals occur as tiny needles or prisms concentrated in plagioclase, and less commonly in mafic minerals. In contrast, apatite occurs as large squat prisms where it has precipitated from high–temperature melt (>۱۰۰۰ oC),

۱۶–The appearance of CIPW normative diopside (Di) that is up to ۸.۴%,

۱۷–The restricted range of ASI values (always ≤۱) with an average of ۰.۹۷ for all analyses,

۱۸–The presence of magnesio–hornblende with appropriate chemical composition led to the recognition that the Kashmar granitoid formed under low temperature, low pressure and oxidized conditions which are the physical states of most I-type granites worldwide, 

۱۹–The allocation of different Kashmar granitoid plutons into a simple suite is based on similarities in field, petrography, isotope and geochemistry. The simple suite commonly represents low temperature I-type granites.

 

Rb/Sr Age Dating of Kashmar Granitoid

In the present work, ۸ samples from major representative plutons of the Kashmar granitoid were selected for Rb–Sr and Sm–Nd analyses (Tables ۱–۲). Biotite flakes was separated from four samples including one alkali feldspar granite, one granodiorite and two granites. The biotites from all dated rocks have been studied under microscope then were analyzed by electron microprobe. They are homogeneous in composition and represent similar characteristics. The Rb/Sr biotite–whole rock isochrones (Figures ۲A–D) for Kashmar plutons yield isotopic ages range from ۴۳.۵ to ۴۲.۴±۰.۴ Ma, representing a typical synchronous plutonism in Middle Eocene times (Lutetian) for emplacement of the Kashmar granitoid. These ages are essentially indistinguishable with age differences of <۱ Ma being negligible for such extensive magmatic suites [cf. ۷]. The obtained ages confirm field observations which indicate that the Kashmar granitoid is younger than the host volcanic rocks.

            Also, the ۸۷Rb/۸۶Sr and ۸۷Sr/۸۶Sr values for all whole rock and separated biotite analyses define an isochron (Fig. ۲E) with a slope that is constrained by biotite samples because they contain significantly higher ۸۷Rb/۸۶Sr and ۸۷Sr/۸۶Sr ratios than whole rock samples. This isochron yields an age of ۴۲.۸±۰.۲ Ma and an initial ۸۷Sr/۸۶Sr ratio of ۰.۷۰۵۴۸±۰.۰۰۰۰۳ both values fall within the range of ages and initial ratios given by different biotite–whole rock isochrones. For each pluton, similarities in age and initial ۸۷Sr/۸۶Sr values indicate that the total rock Rb/Sr system was closed simultaneously, and that the Rb/Sr system has remained a closed system since the time of emplacement.

            The accuracy of the isotopic ages for biotite–whole rock pairs from the Kashmar granitoid is dependant upon the closed system behavior of biotite. Although, Late Palaeogene volcanic activity has been reported in this area, but the Kashmar granitoid was not subjected to isotopic disturbance by younger thermal events. The similarity in isotopic ages from biotite–whole rock pairs clearly records a short duration for the emplacement of different plutons of the Kashmar granitoid in a subvolcanic environment.

 

 

 

Sr and Nd Isotopes in Kashmar granitoid

Isotopic data from different plutons of the Kashmar granitoid (Tables ۱–۲) show low initial ۸۷Sr/۸۶Sr (۰.۷۰۴۷۱–۰.۷۰۵۶۹), intermediate initial ۱۴۳Nd/۱۴۴Nd (۰.۵۱۲۴۸۸–۰.۵۱۲۵۴۸) and negative εNd (–۰.۷۰ to –۱.۸۶) values which are typically within the range of Sr–Nd isotopic signature of I–type granites which form in an infra­–crustal source region. The studied samples show a narrow range of Sm and Nd concentrations (۳.۳۵.۰ ppm and ۱۵.۸–۲۷.۵ ppm, respectively). ۱۴۷Sm/۱۴۴Nd ratios display typical crustal values (۰.۱۰–۰.۱۳) indicating that no major Sm/Nd fractionation took place during magmatic evolution (Fig. ۲F). The εNd values show a narrow range, and for most samples these values are indistinguishable within the ۲s analytical uncertainties (±~۰.۵ units). For each pluton differences between initial ۸۷Sr/۸۶Sr ratios are small but are slightly larger than ۲s analytical uncertainties (±~۰.۰۰۰۰۵). Consequently, a model of simple fractional crystallization of any isotopically uniform melt for the generation of each pluton is unlikely. The isotopic signature of Kashmar granitoid is consistent with low–temperature feature which is mainly distinguishable by presence of zircon crystals in rocks with different SiO۲ contents from each pluton. This implies that zircon was not homogenized in the magma due to the low–temperature condition. Such magmas may produce by partial melting of quartzofeldspathic source rocks and evolve through the combination of restite and fractional crystallization processes [۸a, b].

 

A Typical Simple Suite

The allocation of granites to suites is fundamental to understanding their petrogenetic concepts. Granites from a particular suite may be uniform or varied in chemical composition but they should have distinctive properties, reflecting similar features of their source rocks. Such grouping is well in accord for the Kashmar plutons because of the following features:

۱–These plutons share similarities in field, petrography and geochemistry,

۲–They are equigranular in mineral size, homogeneous in appearance, quartzofeldspathic in nature and contain metaluminous minerals reflecting I–type source composition,

۳–They are similar in age (~۴۲.۵ Ma) although age is not a distinct factor in suite allocation,

۴–They formed under low temperature (۶۱۷–۷۰۰ oC), low pressure (≤۳ kb) and oxidized conditions (log fO۲ = –۱۶ to –۱۹),

۵–They show negative linear trends on Harker plots for most major and trace elements such as P۲O۵, Fe۲O۳, Sr and V indicating fractional crystallization is not the only mechanism for their production,

۶–They are high in Na۲O (۳–۵ wt%), Mn (up to ۱۰۷۰ ppm) and Ba (~۵۰۰ ppm) contents supporting I-type origin which indicates the protolith has not undergone weathering cycle,

۷–They are low in TiO۲ (<۱ wt%), Rb (<۲۱۰ ppm), Cr (<۱۰ ppm), Ni (~۲ ppm) and Sn (<۵ ppm) which means their magmas were felsic low-temperature not cumulative rocks,

۸–They show enrichment in LREE with steep negative slope, depleted in Eu and flat in HREE,

۹–They are low in initial ۸۷Sr/۸۶Sr (۰.۷۰۴۷۱–۰.۷۰۵۶۹) ratios and negative εNd (–۰.۷ to –۱.۹) values representing typical infra-crustal source for the origin of the Kashmar granitoid.

The above similarities and common metaluminous I-type characteristic (ASI≤۱ with an average of ۰.۹۷) precisely suggest a ‘simple suite’ for the Kashmar granitoid. Using modern nomenclature of granites, simple suite corresponds fairly closely to the low-temperature I-type granites [۸a, b].

 

Isotopic Age of Associated Volcanic Rocks

The only isotopic age data for the host volcanic rocks to the north of Kashmar city have been reported by [۱], giving K/Ar ages of ۵۷.۲±۳.۷ and ۴۳.۷±۱.۷ Ma on hornblende and biotite, respectively. A significant age difference is observed between the given isotopic ages of hornblende and biotite. As hornblende has the capacity to be more retentive with respect to ۴۰Ar than biotite, the age of hornblende is interpreted as the emplacement time of host volcanic rocks, whereas the younger age for biotite may be the result of loss ۴۰Ar due to the later thermal event caused by volcanic activity or intrusion of the Kashmar granitoid.

            Tertiary volcanic rocks occur in the Gonabad and Bejestan areas, located to the southern vicinities of the Kashmar granitoid. Bina et al. [۹] determined whole rock K/Ar ages of ۶۱±۲ and ۵۴±۲ Ma for andesitic rocks of the Gonabad and Bejestan areas, respectively. The latter age (۵۴±۲ Ma) which corresponds to Bejestan Andesite is very similar to the isotopic age of hornblende from the host volcanic rocks of the Kashmar granitoid. The age of ۵۷.۲±۳.۷ Ma for Kashmar hornblende reported by [۱] is within the ۲σ analytical uncertainty of the isotopic ages of Gonabad and Bejestan Andesites. These isotopic data indicate that Early Eocene volcanic activity occurred prior to the emplacement of Kashmar granitoid.

 

Eocene Magmatism in NE CIP

In view of the field data, the ages obtained for the Kashmar granitoid support a close genetic connection between host volcanic rocks and granitoid emplacement. The calculated isotopic ages of Kashmar granitoids and isotopic data from the host volcanic rocks indicate that volcanic activity in the Kashmar area is believed to have started approximately ۵۷.۲±۳.۷ Ma, continued through the Early to Middle Eocene and was followed by intrusion of the Kashmar granitoid at ۴۲.۸±۰.۵ Ma, suggesting that magmatic activity occurred over a time interval of ~۱۵ million years. Such a period for intense magmatic activity is not unusual [e.g. ۷, ۱۰]. The precise Rb/Sr ages of the Kashmar granitoid, when taken together with the few reliable published isotopic ages for the host volcanic rocks (۵۷–۴۳ Ma) suggest that these subvolcanic plutons were emplaced after or contemporaneous with widespread volcanic extrusions (mostly andesite) during the Middle Eocene in northeastern CIP. The east–west distribution (~۳۵۰ km) of volcanic and plutonic rocks along the northern parts of the Doruneh Fault may be interpreted to be related to extensional magmatism resulting from the upwelling of large volumes of magma being focused along a structural discontinuity in the northeastern CIP.

 

Conclusions

Rb–Sr and Sm–Nd isotopic data suggest that the Kashmar granitoid emplaced in ۴۳.۵ to ۴۲.۴±۰.۴ Ma, corresponding to Middle Eocene (Lutetian Stage) plutonism that extensively occurred in Iran۱:place>۱:country-region>. The similarity in isotopic ages of different plutons results from several criterion including precise sampling, accurate laboratory works, sensitive technical equipment, appropriate dating system, consanguineous plutons, closed system behavior of biotite and no isotopic disturbance after emplacement. The isotopic data for Kashmar granitoid show low initial ۸۷Sr/۸۶Sr (۰.۷۰۴۷۱–۰.۷۰۵۶۹), intermediate initial ۱۴۳Nd/۱۴۴Nd (۰.۵۱۲۴۸۸–۰.۵۱۲۵۴۸) and negative εNd (–۰.۷۰ to –۱.۸۶) values which are typically within the range of Sr–Nd isotopic signature of I–type granites which originated from partial melting of quartzofeldspathic rocks of lower continental crust (infra-crustal). Field, petrography and geochemical data indicate that magmas of the Kashmar granitoid formed under low-temperature, low-pressure and oxidized conditions. Compositional variation of such magmas may be controlled dominantly through the fractional crystallization in combination with restite separation. The close connection between isotopic ages and initial ratios indicate synchronous intrusions and genetically–related plutons for the Kashmar granitoid.

 

References

۱. Bernhardt U. ۱۹۸۳. Middle Tertiary volcanic rocks from the southern Sabzevar Zone, Khorasan۱:city>, NE۱:state> Iran۱:country-region>۱:place>. Geodynamic Project (Geotraverse) in Iran۱:place>۱:country-region>, Report No ۵۱, ۲۷۷–۲۸۴.

۲. Taheri J. ۲۰۰۱. Geological map of Kashmar, (Scale ۱/۱۰۰,۰۰۰), Geological Survey of northeastern Iran۱:place>۱:country-region>, Mashhad۱:place>.

۳. Karimpour M. H. and Malekzadeh A. ۲۰۰۶. Comparison of the geochemistry of source rocks at Tannurjeh Au–bearing magnetite and Sangan Au–free magnetite deposits, Khorasan Razavi۱:city>, Iran۱:country-region>۱:place>.  Iranian Journal of Crystallography and Mineralogy, No. ۱, pp. ۳–۲۶.

۴. Ishihara S. ۱۹۹۸. Redox status of the phanerozoic granites in the Circume–Pacific Orogenic Belts. The Bruce Chappell Symposium: granites, island arcs, the mantle and ore deposits (Abstract Volume), Australian Geological Survey Organization, Record ۱۹۹۸/۳۳, pp. ۳۴–۳۵.

۵. Soltani A. ۲۰۰۰. Geochemistry and geochronology of I-type granitoid rocks in the northeastern Central Iran Plate. Ph.D. Thesis (unpubl.), University of Wollongong۱:city>, Australia۱:country-region>۱:place>, ۳۰۰ p.

۶. Turner S. P., Foden J. D. and Morison R. S. ۱۹۹۲. Derivation of some A-type magmas by fractionation of basaltic magma: an example from the Padthaway Ridge, South Australia. Lithos, ۲۸, pp. ۱۵۱-۱۷۹.

۷. Nakajima T. ۱۹۹۶. Cretaceous granitoids in SW Japan۱:place> and their bearing on the crust-forming process in the eastern Eurasian margin. Transactions of the Royal Society of Edinburgh۱:place>۱:city>: Earth Science, ۸۷, pp. ۱۸۳-۱۹۱.

۸a. Chappell B. W. ۲۰۰۴. High– and Low–temperature Granites. The Ishihara Symposium: Granites and Associated Metallogenesis, GEMOC, Macquarie۱:placename> University۱:placetype>۱:place>, NSW, ۲۰۱۰, Australia۱:place>۱:country-region>, pp. ۴۳.

۸b. Chappell, B. W. ۲۰۰۴. Towards a unified model for granite genesis. Transactions of the Royal Society of Edinburgh: Earth Sciences, v. ۹۵, pp.۱-۱۰.

۹. Bina M. M., Bucur۲:sn> I.۲:sn>۱:place>, Prevot M., Meyerfeld Y., Daly L., Cantagrel J. M. and Mergoil J. ۱۹۸۶.  Paleomagnetism, petrology and geochronology of Tertiary magmatic and sedimentary units from Iran۱:place>۱:country-region>. Tectonophysics, v. ۱۲۱, pp. ۳۰۳–۳۲۹.

۱۰. Milner S. C., Le Roex A. P., and Watkins R. T. ۱۹۹۳. Rb/Sr age determinations of rocks from the Okenyenya igneous complex, northeastern Namibia۱:place>۱:country-region>. Geological Magazine, v. ۱۳۰, pp. ۳۳۵–۳۴۳.

 

 

ابوالفضل سلطانی

(Ph.D. در ژئوشیمی و ژئوکرونولوژی گرانیتها از استرالیا)

 عضو گروه عمران دانشکده فنی و مهندسی دانشگاه تربیت دبیر شهید رجایی، تهران

 

چکیده:

گرانیتوئید کاشمر ~200 km2)) واقع در شمال شرقی صفحه ایران مرکزی، بخش بزرگی از کمربند ماگمایی شمال گسل درونه است که در سنگهای ولکانیکی ائوسن آغازین نفوذ کرده و با رخنمونی وسیع از پلوتونهای I–type که عمدتاً دارای سنگهای فلسیک از نوع گرانیت و گرانودیوریت هستند، یک سایت ماگمایی ساده (simple suite) را با خواص گرانیتهای حرارت پایین و فشار کم در محیطی ساب ولکانیک نشان میدهد. تعیین سن بیوتیت–کل سنگ به روش Rb/Sr روی نمونه هایی از گرانیت، گرانودیوریت و آلکالی فلدسپار گرانیت محدوده سنی 43.5 تاMa  0.4±42.4 را بدست میدهد که بیانگر ائوسن میانی (لوت تین) و جایگزینی طیّ دوره ای بسیار کوتاه برای پلوتونهای گرانیتوئید کاشمر است. ژئوشیمی ایزوتوپهای Sr و Nd همگی با تغییرات محدود، حاکی از پایین بودن نسبتهای ایزوتوپ منشأ برای87Sr/86Sr ، مقادیر متوسّط برای 143Nd/144Nd و مقادیر منفی εNd در تمامی آنالیزهای گرانیتوئید کاشمر است. بر اساس مطالعات ایزوتوپی و پترولوژیکی ماگماهای سازنده گرانیتوئید کاشمر با ساختار ژنتیکی بسیار مشابه، از ذوب سنگهای کوارتز و فلدسپاری در پوسته تحتانی منشأ گرفته و طیّ فرایندهای تفریق توٲم با جدایش رستایت تحوّل یافته اند.

 

Rb/Sr biotite–whole rock dating from Kashmar granitoid

 

Abolfazl Soltani

(Ph.D. in geochemistry and geochronology of granites, Australia)

Faculty of Civil Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran

 

Abstract:

The Kashmar granitoid (~200 km2) locates in the northeastern Central Iran Plate (CIP). It is a major plutonic mass occurring in North Doruneh Fault Magmatic Belt. It intrudes into Early Eocene volcanic rocks and extensively crops out by felsic I-type plutons which are mainly granite and granodiorite in composition. These plutons share several characteristics which indicate that the Kashmar granitoid is a simple magmatic suite, formed under low temperature and low pressure conditions in a sub–volcanic environment. Rb/Sr dating on biotite–whole rock pairs from granite, granodiorite and alkali feldspar granite yield isotopic ages ranging from 43.5 to 42.4±0.4 Ma which indicate Middle Eocene times (Lutetian) and a short time interval for emplacement of plutons of the Kashmar granitoid. The Sr and Nd isotopes represent low initial 87Sr/86Sr, intermediate initial 143Nd/144Nd and negative εNd values for all analyses of the Kashmar granitoid. Isotopic and petrological studies show that magmas of the Kashmar granitoid originated by partial melting of quartzofeldspathic source rocks from lower continental crust (infra-crustal) and evolved dominantly through the restite separation in combination with fractional crystallization processes.

کلید واژه ها: خراسان رضوى