Garnet compositions and their use as indicators of peraluminous granitoid petrogenesis — southeastern Arabian Shield

Garnet, an uncommon accessory mineral in igneous rocks, occurs in seven small peraluminous granitoid plutons in the southeastern Arabian Shield; textural equilibrium between garnet and other host granitoid minerals indicates that the garnets crystallized from their host magmas. Compositions of the garnets form three groups that reflect host-granitoid compositions, which in turn reflect source compositions and tectonic regimes in which the host magmas were generated. Garnets from the seven plutons have almandine-rich cores and spessartine-rich rims. This reverse zoning depicts host magma compositional evolution; i.e. rimward spessartine enrichment resulted from progressive, host-magma manganese enrichment. The garnets are heavy rare-earth element enriched; (Lu/La) _N ranges from 13 to 355 and one of the garnets contains spectacularly elevated abundances of Y, Ta, Th, U, Zn, Zr, Hf, Sn, and Nb. Involvement of garnets with these trace element characteristics in magma genesis or evolution can have dramatic effects on trace element signatures of the resulting magmas. Other researchers suggest that Mn-enriched magmas are most conducive to garnet nucleation. Although the garnetiferous granitoids discussed here are slightly Mn enriched, other genetically similar peraluminous Arabian granitoids lack garnet; Mn enrichment alone does not guarantee garnet nucleation. The presence of excess alumina in the magma may be a prerequisite for garnet nucleation.     

The evolution of the Arabian lower crust and lithospheric mantle — Geochemical constraints from southern Syrian mafic and ultramafic xenoliths

Geochemical compositions of lower crustal and lithospheric mantle xenoliths found in alkali basaltic lavas from the Harrat Ash Shamah volcanic field in southern Syria place constraints on the formation of the Arabian–Nubian Shield in northern Arabia. Compositions of lower crustal granulites are compatible with a cumulate formation from mafic melts and indicate that they are not genetically related to their host rocks. Instead, their depletion in Nb relative to other incompatible elements points to an origin in a Neoproterozoic subduction zone as recorded by an average depleted mantle Sm–Nd model age of 630 Ma.Lithospheric spinel peridotites typically represent relatively low degree (< 10%) partial melting residues of spinel lherzolite with primitive mantle compositions as indicated by major and trace element modelling of clinopyroxene and spinel. The primary compositions of the xenoliths were subsequently altered by metasomatic reactions with low degree silicate melts and possibly carbonatites. Because host lavas lack these signatures any recent reaction of the lherzolites with their host magma can be ruled out. Sm–Nd data of clinopyroxene from Arabian lithospheric mantle lherzolites yield an average age of 640 Ma suggesting that the lithosphere was not replaced since its formation and supporting a common origin of the Arabian lower crustal and lithospheric mantle sections.The new data along with published Arabian mantle xenolith compositions are consistent with a model in which the lithospheric precursor was depleted oceanic lithosphere that was overprinted by metasomatic processes related to subduction and arc accretion during the generation of the Arabian–Nubian Shield. The less refractory nature of the northern Arabian lithosphere as indicated by higher Al, Na and lower Si and Mg contents of clinopyroxenes compared to the more depleted nature of the south Arabian lithospheric mantle, and the comparable low extent of melt extraction suggest that the northern Arabian lithosphere formed in a continental arc system, whereas the lithosphere in the southern part of Arabia appears to be of oceanic arc origin.     

Fe-Li云母化学成分的解释和分类

用置换矢量概念解释了115个天然 Fe-Li 云母化学成分的变化。Fe-Li 云母是三八面体 Li-Fe-Al 云母,其基本置换是四锂云母置换。由于 Al-Li 白云母置换和白云母置换的影响,其化学组成变化的基本趋势呈明显的非线性,因而 Fe-Li 云母不是真正的二元系。作为 Fe-Li 云母,富铁黑云母和铁叶云母都是最富铁的成员,因而建议称 Fe-Li 云母为黑云母-锂云母系列。根据化学成分,晶胞参数和折光率的异常变化还提出了该系列自然分类的方案。     

Dehydration melting of micas in the Chilka Lake Khondalites: The link between the metapelites and granitoids

Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes and spinels from the metapelites are compatible with those in the bulk rocks. A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived from dehydration melting experiments of micas in pelitic and psammitic rocks. The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na₂O-K₂O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting. Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different compositions. The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment of restites is considered to be the main causative factor. Disequilibrium in terms of major elements is illustrated by leucosomes within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental melts approach equilibrium melt compositions closely.     

Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ ¹⁸O_(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault. Received: 26 February 1999 / Accepted: 8 November 1999     

Distribution of oceanic and continental leads in the Arabian-Nubian Shield

New common lead data for feldspar, whole-rock, and galena samples from the Arabian-Nubian Shield, together with data from previous work, can be divided into two main groups. Group I leads have oceanic (mantle) characteristics, whereas group II leads have incorporated a continental-crustal component of at least early Proterozoic age. The group I leads are found in rocks from the Red Sea Hills of Egypt and the western and southern parts of the Arabian Shield. Group II leads are found in rocks from the northeastern and eastern parts of the Arabian Shield, as well as from the southeastern Shield near Najran. They are also found in rocks to the south in Yemen, to the east in Oman, and to the west at Aswan, Egypt. This distribution of data suggests that the Arabian-Nubian Shield has an oceanic core flanked by rocks that have developed, at least in part, from older continental material. Two mechanisms are suggested by which this older lead component could have been incorporated into the late Proterozoic rocks, and each may have operated in different parts of the Shield. The older lead component either was derived directly from an underlying early Proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent.New U-Pb zircon data indicate the presence of an early Proterozoic basement southeast of Jabal Dahul in the eastern Arabian Shield. These data, together with 2,000-Ma-old zircons from the Al Amar fault zone, verify the implication of the common lead data that at least a part of the eastern Arabian Shield has an older continental basement.Because continental margins are particularly favorable locations for development of ore deposits, these findings may have important economic implications, particularly for tin, tungsten, and molybdenum exploration.     

Trace element geochemistry by laser ablation ICP-MS of micas associated with Ta mineralization in the Tanco pegmatite,Manitoba, Canada

In the Tanco pegmatite, one of the world’s major Ta deposits, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Micas are common in most of the pegmatite units and are intimately associated with the successive stages of Ta mineralization, from the wall zone to the central zones where micaceous replacement is pervasive. Different generations of micas, both primary and secondary, associated with Ta oxides, were selected for electron microprobe and laser ablation ICP-MS investigation. Their chemical trends are used to constrain the magmatic versus hydrothermal processes that played a role in their crystallization and their associated Ta mineralization. Micas range from dioctahedral muscovite to trioctahedral lepidolite through Al↔Li substitution. Unexpectedly, the most evolved compositions (low K/Rb ratios and high Li contents) occur in the wall zone; they are interpreted to reflect nonequilibrium crystallization from an undercooled melt, with or without boundary layer effects. In the central zones, the fine-grained mica–quartz assemblage hosts some coarser-grained Li-muscovite, which has the highest Ta contents (up to 400 ppm). These Li–F–a-rich micas are interpreted to originate from a magmatic metasomatic event, which was also at the origin of the MQM-style Ta mineralization at Tanco. However, the Li–Ta-poor, muscovite end-member compositions of fine-grained alteration micas suggest crystallization from an aqueous fluid, during a metasomatic (hydrothermal) event involving late pegmatitic fluids. The low Ta concentrations (around 50 ppm) of this fine-grained muscovite suggest that this fluid transported at least small amounts of Ta.     

From the Libyan border to the Nile – Neoproterozoic magmatism and basement evolution of southern Egypt

ABSTRACT

In southern Egypt, Neoproterozoic granitoids intrude the crust from Libya in the west to the Nile River in the east. Best exposed at Gebel Uweinat, Gebel Kamil, Bir Safsaf, Gebel El Asr and Gebel Umm Shaghir, these rocks represent the transition between the rejuvenated Archean to Paleoproterozoic crust of the east Saharan craton and the juvenile Neoproterozoic crust of the Arabian–Nubian Shield to the east. We present U-Pb zircon geochronology and whole rock geochemistry from 16 Neoproterozoic (c. 628–580 Ma) granitoids across southern Egypt which document the synchronous production of diverse compositions indicative of an intraplate or intracontinental tectonic setting. Our data better constrain the age relationships, petrogenesis, and Pan-African crustal growth associated with the amalgamation of Gondwana and contribute to understanding the crust-modifying processes that led to the development of the Saharan metacraton in the Ediacaran.     

Secondary Ca-Al silicates as low-grade alteration products of granitoid biotite

Secondary Ca-Al silicates, including andradite-grossular garnet, epidote, pumpellyite, and prehnite are shown to be extremely widespread as low-grade alteration products of granitoid biotite. All may occur within metadomains on the scale of a single biotite pseudomorph. They all generally have a lensoid habit parallel to the host biotite cleavage, which may, or may not be deformed. A cleavage or parting inherited from the host biotite cleavage is usually present.Prehnite, pumpellyite and epidote are unusually Fe-rich: 100 Fe/Fe + Al for prehnite and pumpellyite reaching 19.1 and 49.1 respectively and epidote Ps₃₆-Ps₄₈. Garnet compositions are relatively uniform averaging approximately andradite₅₆ grossular₃₅ hydrogrossular₇.Correlation of prehnite Fe³⁺ with host biotite Fe³⁺ and oxidation state support the textural evidence that prehnite in this paragenesis often replaces the host biotite. Sericitization of plagioclase and commonly a complete lack of hornblende indicate that plagioclase (An₅₀-An₁₅) is the chief source of Ca. Chlorite and muscovite (whose compositions are both directly related to the host biotite composition), aluminian sphene (Al₂O₃ reaching 7.72%), and K-feldspar (Or₉₉₊) are complimentary to the biotite breakdown reaction.Host rock composition to some extent controls the development of the Ca-Al silicates which do not occur in granitoids whose whole-rock CaO is less than 1%. An aqueous pore fluid with locally varying activities of ions enabled concentration of Ca in micro-metadomains, allowing development of Ca-Al silicates in relatively low-Ca granitoids. Individual phases generally appear to have developed independently due to fluctuation in chemical environment in micro-space and/or T, P fluctuation with time. Textures suggest that grandite may have locally replaced epidote in response to increasing temperature.In granitoids of the Victoria Range, South Island, New Zealand the alteration has probably occurred with P<2kb and T< 300 °–350 °C. Here alteration can be ascribed to strictly deuteric activity in only one of the two groups of granitoids so affected.     

Ediacaran terrane accretion within the Arabian–Nubian Shield

The Ad Dawadimi Terrane is an Ediacaran basin of the Arabian Nubian Shield (ANS), Saudi Arabia. This basin terrane is situated in the far eastern part of the ANS and represents the youngest accretion event of the exposed ANS. Therefore, the timing of events within the basin is key to understanding both the closure of the Mozambique Ocean and the amalgamation of Gondwana along the northern East African Orogen. Here we present U/Pb detrital zircon data for the Abt Formation, the principle basin sediments of the Ad Dawadimi Terrane, along with ⁴⁰Ar/³⁹Ar ages on muscovite and whole rock Sm/Nd data. These data indicate that deep-water deposition in the Abt Basin did not end until after ca. 620 Ma and that deformation and greenschist-facies metamorphism of the Abt Formation occurred at 620 ± 3 (2σ) Ma along an active margin. This is the youngest terrane amalgamation event reported so far in the Arabian–Nubian Shield, but we suggest even younger sutures lie further east beneath the Phanerozoic cover of eastern Saudi Arabia. Our results suggest that the Ediacaran basins of the eastern ANS were not part of the Huqf basin in Oman, which was instead part of a passive margin of Neoproterozoic India, separated from the active margin of Africa by the Mozambique Ocean that probably did not close until the late Ediacaran or early Cambrian.     

Mg-Fe云母化学成分的解释和分类(Ⅱ)——Mg-Fe云母的自然分类

给出云母化学成分比值FM=6(Fe⁺²+Mn)/(Fe²⁺+Mn+Mg),用以表示Mg-Fe云母成分变化的基本趋势。根据291个云母的化学成分相对于FM值的异常变化,提出了天然Mg-Fe云母自然分类的新方案:把FM≌1.20(1.05-1.35)、4.80作为区分金云母、黑云母和铁叶云母的界线;FM>0.60的金云母称为铁金云母;以FM=2.70、3.60把黑云母再细分为富镁黑云母、镁铁黑云母和富铁黑云母。Mg-Fe云母自然分类的本质是各成员云母之间白云母置换方式和限度的潜在差异在不同的自然结晶条件下的反映。     

Geochemical constraints on crustal anatexis: a case study from the Pan-African Damara granitoids of Namibia

 Major and trace element models of recently published vapour-absent mica dehydration melting experiments are used to identify granitoids generated by muscovite and biotite dehydration melting, and to distinguish between plagioclase-limited and biotite-limited, biotite dehydration melting. In the case of granitoids from the Pan-African Damara mobile belt (Namibia), many of the leucogranites and Salem-type granitoids may be modelled by biotite dehydration melting. The low Rb/Sr granitoids (e.g. Donkerhuk Onanis, Salem Onanis, Donkerhuk Nomatsaus, Salem Goas) probably reflect feldspar-limited, biotite dehydration melting (a pelitic source) whereas the high Rb/Sr suites (e.g. Bloedkoppie leucogranite, Stinkbank leucogranite, Salem Swakopmund, Leucocratic Stink bank granite) reflect biotite-limited, biotite dehydration melting (a greywacke source). Alaskites from the Damara belt have major element compositions which are consistent with muscovite dehydration melting, and their positive Eu anomalies are linked to high K₂O reflecting K-feldspar entrainment. Combined Zr and LREE (light rare earth element) solubility models indicate that insufficient time (probably less than 10⁴ years) had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium concentrations of Zr and the LREEs. In contrast, the leucogranites and Salem-type granites have attained their equilibrium inventories of these trace elements. Combined Fe₂O₃ and MgO contents in some samples from two granitoids (the Salem Goas and Donkerhuk Onanis intrusions) are higher than those readily attainable by biotite dehydration melting indicating either: (1) that they contain a contribution from melts generated by incipient garnet breakdown or; (2) that they contain small amounts of an entrained ferromagnesian phase. Received: 24 April 1995/Accepted: 11 December 1995     

Internal differentiation of rare-element pegmatites: Effects of boron,phosphorus, and fluorine

Lithium-rich, rare-element pegmatites are characterized by high concentrations of B, P, and F. The interactions of these components with H₂O and rare alkalis lower liquidas and solidus temperatures, enhance silicate liquid-H₂O miscibility, and control partitioning and concentration of Group I elements and higher-field-strength cations. Boron, F, and perhaps P may form peralkaline Na- and Li-species that promote early saturation in mica + quartz. Activities of F, P, and especially B are largely unbuffered throughout crystallization. Concentration of these components through fractional crystallization involving muscovite generates a peralkaline, Na-aluminosilicate-rich melt or vapor from which albitites rich in tourmaline, phosphates, F-rich micas, beryl, zircon, and Nb-Ta-Sn oxides crystallize. Phase equilibrium experiments with peraluminous B-P-F-rich rhyolite obsidian (macusanite) simulate many features of rare-element pegmatites, especially at H₂O-undersaturated conditions.     

Gold mineralizations in the Arabian shield: statistical control on mining potential

Saudi Arabia possesses numerous low-grade gold deposits and many occurrences are located in the Arabian Shield. Currently, there are five operating gold mines and major plans are underway to develop three gold mining regions in various parts of the shield. Because of the presence of numerous deposits and a significant amount of investments earmarked for the development of these deposits by the Saudi Government during the last few years, Saudi Arabia is expected to become one of the leading gold producers in the world. This paper starts with an introduction of gold mineralization and mining activities in the Arabian Shield in a historical perspective. This is followed by a brief review of geology and the geological importance of the Arabian Shield as a host for various types of gold deposits. The latter part of the paper discusses the statistical distribution of gold grades and its impact on cutoff grade variations on the gold reserves and their mining potentials. Finally, considering the lognormal distribution of gold grades and reduction in cutoff grades, it is demonstrated how some of the previously classified uneconomic resources can be gradually converted into mineable reserves with increasing proportions.     

Characterization of non-equilibrium and equilibrium occurrences of paragonite/muscovite intergrowths in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy)

 Coexisting muscovite and paragonite have been observed in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy). The P-T conditions of this eclogite reached 570–650 °C and 19–21 kbar and the rocks show several stages of mineral growth during their retrograde path, ranging from the subsequent lower-P eclogite facies to the blueschist facies and then the greenschist facies. Muscovite and paragonite are very common in these rocks and show two texturally different occurrences indicating equilibrium and non-equilibrium states between them. In one mode of occurrence they coexist in equilibrium in the lower-P eclogite facies. In the same rock muscovite ± albite also replaced paragonite during a greenschist-facies overprint, as evidenced by unique across – (001) layer boundaries. The chemical compositions of the lower-P eclogite-facies micas plot astride the muscovite – paragonite solvus, whereas the compositions of the greenschist-facies micas lie outside the solvus and indicate disequilibrium. The TEM observations of the textural relations of the greenschist-facies micas imply structural coherency between paragonite and muscovite along the layers, but there is a sharp discontinuity in the composition of the octahedral and tetrahedral sheets across the phase boundary. We propose that muscovite formed through a dissolution and recrystallization process, since no gradual variations toward the muscovite – paragonite interfaces occur and no intermediate, homogeneous Na-K phase has been observed. Because a solid-state diffusion mechanism is highly unlikely at these low temperatures (300–500 °C), especially with respect to octahedral and tetrahedral sites, it is assumed that H₂O plays an important role in this process. The across-layer boundaries are inferred to be characteristic of such non-equilibrium replacement processes. The characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition and therefore about the state of equilibrium between both micas, which in turn may lead to misinterpretations of thermometric results. Received: 3 February 1999 / Accepted: 19 October 1999     

Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO₂, Na₂O + K₂O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.     

Neoproterozoic calc‐alkaline peraluminous granitoids of the Deleihimmi pluton,Central Eastern Desert,Egypt: implications for transition from late‐ to post‐collisional tectonomagmatic evolution in the northern Arabian–Nubian Shield

The widely distributed late‐collisional calc‐alkaline granitoids in the northern Arabian–Nubian Shield (ANS) have a geodynamic interest as they represent significant addition of material into the ANS juvenile crust in a short time interval (∼630–590 Ma). The Deleihimmi granitoids in the Egyptian Central Eastern Desert are, therefore, particularly interesting since they form a multiphase pluton composed largely of late‐collisional biotite granitoids enclosing granodiorite microgranular enclaves and intruded by leuco‐ and muscovite granites. Geochemically, different granitoid phases share some features and distinctly vary in others. They display slightly peraluminous (ASI = 1–1.16), non‐alkaline (calc‐alkaline and highly fractionated calc‐alkaline), I‐type affinities. Both biotite granitoids and leucogranites show similar rare earth element (REE) patterns [(La/Lu)_N = 3.04–2.92 and 1.9–1.14; Eu/Eu* = 0.26–0.19 and 0.11–0.08, respectively) and related most likely by closed system crystal fractionation of a common parent. On the other hand, the late phase muscovite granites have distinctive geochemical features typical of rare‐metal granites. They are remarkably depleted in Sr and Ba (4–35 and 13–18 ppm, respectively), and enriched in Rb (381–473 ppm) and many rare metals. Moreover, their REE patterns show a tetrad effect (TE_1,3 = 1.13 and 1.29) and pronounced negative Eu anomalies (Eu/Eu* = 0.07 and 0.08), implying extensive open system fractionation via fluid–rock interaction during the magmatic stage. Origin of the calc‐alkaline granitoids by high degree of partial melting of mafic lower crust with subsequent crystal fractionation is advocated. The broad distribution of late‐collisional calc‐alkaline granitoids in the northern ANS is related most likely to large areal and intensive lithospheric delamination subsequent to slab break‐off and crustal/mantle thickening. Such delamination caused both crustal uplift and partial melting of the remaining mantle lithosphere in response to asthenospheric uprise. The melts produced underplate the lower crust to promote its melting. The presence of microgranular enclaves, resulting from mingling of mantle‐derived mafic magma with felsic crustal‐derived liquid, favours this process. The derivation of the late‐phase rare‐metal granites by open system fractionation via fluid interaction is almost related to the onset of extension above the rising asthenosphere that results in mantle degassing during the switch to post‐collisional stage. Consequently, the switch from late‐ to post‐collisional stage of crustal evolution in the northern ANS could be potentially significant not only geodynamically but also economically. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-stoichiometry of interlayer cations in micas from low- to middle-grade metamorphic rocks in the Ryoke and the Sanbagawa belts,Japan

Micas in 17 pelitic (K-feldspar-free) and 8 psammitic (K-feldspar-bearing) rocks from the Wazuka and the Asemi-gawa areas in the Ryoke and the Sanbagawa metamorphic terrains, respectively, were analyzed on an electron-probe microanalyzer. The deficiency of alkali cations in the low- to middle-grade metamorphic micas is ascribed to the illite substitution, K^XII+Al^IV=^XII (vacancy)+Si^IV.At the same metamorphic grade, the deficiency of interlayer cations in micas from the pelitic rocks is greater than that from the psammitic rocks. However, it decreases with rising temperature in both rock-types, irrespective of the pressure of metamorphism.K-feldspar and biotite buffer the illite substitution. Two reactions are proposed to explain the decrease of the alkali-cation deficiency in both muscovite and biotite.     

The origin of variable-δ18O zircons in Jurassic and Cretaceous Mo-bearing granitoids in the eastern Xing–Meng Orogenic Belt,Northeast China

This study reports new zircon U–Pb ages, Lu–Hf isotope data, and oxygen isotope data for Mesozoic Mo-bearing granitoids in the eastern Xing–Meng Orogenic Belt (XMOB) of Northeast China, within the eastern Central Asian Orogenic Belt. Combining these new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb ages with the results of previous research indicates that two stages of Mo-bearing granitoid magmatism occurred in the eastern XMOB, during the Early–Middle Jurassic (200–165 Ma) and the Early Cretaceous (ca. 111 Ma). The eastern XMOB also contains Mo-bearing granitoids with variable δ¹⁸O compositions that record variations in source oxygen isotopic compositions. Combining δ¹⁸O data with zircon U–Pb and Hf isotopic data provides evidence of the origin of these granitoids. Three types of zircon have been identified within these granitoids. Type 1 zircons formed during the Mesozoic and having high δ¹⁸O values (5.71–7.05‰) that are consistent with the compositions of magmatic zircons from the Luming, Jiapigou, and Kanchuangou areas. These zircons suggest that the Mo-bearing granitoids were derived from a source containing supracrustal materials. The type 2 zircons have extremely low and heterogeneous δ¹⁸O values (4.64–4.89‰) that are consistent with the compositions of magmatic zircons from the Jidetun and Fuanpu areas. These magmas were generated by the remelting of juvenile crustal material that was previously significantly modified by interaction with fluids. Type 3 zircons generally have mantle-like δ¹⁸O values (5.42–5.57‰), with several zircons yielding higher δ¹⁸O values, suggesting that these intrusions formed from mantle-derived magmas that assimilated and were metasomatized by crustal material. Combining these geochemical data with the geology of this region indicates that the Mo-bearing granitoids were generated as a result of subduction of the Palaeo-Pacific Plate beneath the Eurasian continent.     

天山0．9Ga新元古代花岗岩SHRIMP锆石U-Pb年龄及其构造意义

天山造山带的中天山隆起带中广泛出露着新元古代花岗岩类岩石。获得了东天山星星峡和西天山温泉地区片麻状花岗岩中锆石的SHRIMP U-Pb年龄,分别为（942±7）Ma（2σ）和（919±6）Ma（2σ）。这些花岗岩以特有的粗粒、巨大的眼球状片麻结构为特征。主元素数据表明它们属于过铝质的二长花岗岩,具有高SiO2（≥170％）、高K2O＋Na2O（7％～≥8％）,且K2O〉Na2O。这些新元古代花岗岩类岩石具不同的REE含量,但它们有近于平行的REE分布曲线和明显的Eu负异常,同时代的星星峡混合岩具有较高的REE含量,明显富集LREE。它们的微量元素蛛网图几乎完全相同,均有明显的Nb、Sr、Ti、P负异常。基于Nd同位素组成研究,其εNd（t）在-4至-0之间,Nd模式年龄tDM为2．0～1．6Ga。由于放射成因Sr同位素组成非常高,不可能得到精确有意义的Sr同位素初始值。上述特征均表明这些新元古代花岗岩形成于大陆边缘构造环境,由古元古代地壳重熔并经历了高度的结晶分异作用而形成。综合近年来发表的关于天山各地段,诸如温泉、赛里木湖以东、拉尔敦达坂、巴仑台和星星峡等地区较为可靠的锆石U—Pb年龄数据,我们认为天山新元古代花岗岩类岩石主要形成于960-910Ma。结合塔里木盆地周边古老地块年龄的研究结果,可以推断在早新元古代时期塔里木周边和天山的古老地块可能曾组成Rodinia超大陆的一部分。