Abstract: Microgabbrodiorite, microdiorite and less than microgranodiorite enclaves occur in the Astaneh pluton. These enclaves have I-type mineral assemblages that are broadly similar to those in the host granitoids except for the greater abundance of mafic minerals, such as amphibole. They show various features formed by magma mixing/mingling environment: abundant subrounded shape, sharp but partly diffuse contact with host granitoids, finer grain size than host granites, more mafic small enclave in large enclave, ocellar quartz, acicular apatite, poikilitic textures and pargasitic amphibole in dacitic enclave. Indeed geochemistry evidences and enclaves normalized against their host granodiorites show that magma mingling occurred.
INTRODUCTION
The study of enclaves is important because it can shed light on the mode of emplacement of granitoid magmas, the nature of the country rocks, the dynamics of magma chambers and cooling plutons and the origin and evolution of granitoid magmas (Barbarin and Didier, 1991).
The microgranular enclaves hosted in granitoids may serve as a potential tool to understand the process of mafic and felsic magma interaction, where microgranular enclaves can represent pristine mafic magma or a hybrid (mafic and felsic) product mingled in convective, calc-alkaline plutonic systems (Castro et al., 1990; Wiebe et al., 1997). The typical igneous texture described by Vernon (1991) and fine-grained margins surrounding many MME (Mafic Microgranular Enclaves) favor their formation from the crystallization of magma.
The three main theories for the petrogenesis of microgranular enclaves interpret them as: 1 fragments of wallrock facies closely related to the host magma or of cognate fragments of cumulates; 2 globules of more mafic magma, generally hybrid magma, comingled with more felsic host magma and 3 fragments of recrystallized, refractory metamorphic rocks and fragments of melt residues from the granite source as summarized by Maas et al. (1997). So clearly the origin of microgranular enclaves is still debated; the most popular choice is the magma mixing and mingling models. Magma mixing causes homogenization of the interacting melt phases and the conversion of early crystals to partly dissolved (corroded) in new hybrid magma, whereas, mingling or co-mingling involves partial mixing or interpenetration of felsic−mafic magmas without pervasive changes (Vernon, 1983; Sparks and Marshall, 1986; Barbarin and Didier, 1992). However, magma mingling (either as cogenetic mafic and felsic magmas or as two coeval magmas) as a mechanism for the genesis of MME, which are common in I-type granitoid rocks, have received the most attention from recent researchers (Didier, 1987; Vernon et al., 1988; Dodge and Kistler, 1990; Barbarin and Didier, 1991; Castro et al., 1991; Fershtater and Borodina, 1991; Tobisch et al., 1997; Sinha et al., 2001; Dahlquist, 2002). We will employ the term "mixing when homogenous hybrid rocks are formed and the term mingling or Co-mingling according to Vernon (1990) and Barbarin and Didier (1992), should be restricted to systems with only mechanical interactions. The term hybridization will refer to compositional modifications of the two magmas.
This study presents the mineralogy, petrography, major and trace element geochemistry of microgranular enclaves from the Astaneh area, Sanandaj-Sirjan Zone, Western Iran (Fig. 1). The geochemical data are used to test specific magmatic processes which may have been involved in the genesis of the microgranular enclaves.
MATERIALS AND METHODS
A total of about 50 samples from different enclaves including gabbro dared, dared, tonalite and granodiorite collected. Fifty thin sections of these samples were prepared and studied by optical microscope and 5 thin polished sections were selected for electron microprobe. Representative samples (11 samples) were then selected for whole rock geochemistry. Sample weights were 1-1.5 kg before crushing and powdering. Major, minor and trace element abundances were determined by X-Ray Fluorescence (XRF), using a Borker S4 Pioneer Spectrometer and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) at the University of Huelva in March 2006 during 6 months, Spain. XRF analysis used approximately 0.1 g rock powder that was first mixed with 0.9 g Li2B4O7 in a crucible and then melted in 1150°C. Trace and REE analysis by ICP_MS, in this way used 0.1 g powder that mixed to HF, HCl and HNO3 in Teflon vials, the analytical processes are described by Cotton et al. (1995). Relative standard deviations were ≤±2% for major elements and ≤±5% for trace elements. The results of the analyses are reported in Table 1. The major element compositions of the minerals were determined by Electron Microprobe analysis of polished thin sections.
| Table 1: | Whole-rock geochemical compositions from the Enclaves and average of host grarnite of the Astaneh pluton |
The analyses were performed with JXA-8200 Super Probe at university of Huelva (Spain), operated with an accelerating voltage 15 keV and a probe current of 5 nA. Silicate standards were Jadeite for Na, wollastonite for Ca, Alkali feldspar for K and Al, enstatite for Mg, fayallite for Fe and Mn and apatite for P.
RESULTS
Geological setting: The Astaneh pluton is a NNW-SSE trending body covering an area of 30 km2, approximately 10 km in length and 3 km in width, which lies between 33° 30`-34°N and between 49°15`-49°, 25` E (Fig. 1b). The Astaneh area is characterized by the predominance of metamorphic rocks of Jurassic age (Baharifar, et al., 2004) and the presence of the Astaneh pluton. Metamorphic rocks subdivided to 2 groups based on their setting: Dynamothermal and Contact. Dynamotermal metamorphism has affected a vast area and is composed of slate, phyllite and schist (Ahmadi et al., 2007). By the injection of the Astaneh pluton, a contact metamorphism has occurred which can be considered as an albite-epidote hornfels facies. Contact metamorphism rocks consist of spotted schist, hornfels schist and honfels (Ahmadi et al., 2007). The compositional variation found in this major pluton, usually range from quartz-dared-tonalite to monzogranite and subvolcanic rocks to rhyodacite composition.
| Fig. 1: | (a) Geological map of Iran (modified from: Shahabpour,
1994), showing major lithotectonic unites and (b) the Astaneh area,
western Iran |
| Fig. 2: | SiO2 versus Na2O+K2O diagram of the Astaneh granitoids (Middlemost, 1985) |
However, dared and more basic rocks are included large xenoliths in this pluton. A very common feature of Sanandaj-Sirjan Zone granitic type is conspicuous presence of mafic microgranular enclave dispersed, especially in the granodiorites and monzogranites of Astaneh. The granitoids occurring in Astaneh show close similarities with those described elsewhere in the Sanandaj-Sirjan Zone, exceptionally occurrence subvolcanic rocks in Astaneh. In general, mineral assemblage in Astaneh pluton and its related subvolcanic is same to other calc-alkaline granites in Sanadaj-Sirjan Zone.
Petrography and mineral chemistry
Host rock granitoids: The granitoids are variable in grain size,
with the principal rock forming minerals ranging from 0.2 to 5.0 mm in
diameter. Tonalities and granodiorites are occasionally plagioclase porphyritic
whereas granites may have phenocrysts of both plagioclase and quartz.
Plagioclase is usually subhedral but can be euhedral, especially when
occurring as chadacrysts enclosed by quartz or alkali-feldspar. Quartz
is always anhedral and occurs in consertal aggregates. Alkali-feldspar
is also anhedral in tonalities and granodiorites, occupying interstices
or partially mantling plagioclase. Biotite varies in morphology from anhedral
patches, sometimes after amphibole, to subhedral crystals. Amphibole is
euhedral to subhedral and occupies interstices between plagioclases. Apatite,
zircon and rare sphene are accessory phases.
Enclaves: The enclaves are oval or irregular-shaped masses ranging in size from mm to meters. They appear regularly as inclusions within the granodiorite-monzogranite series. Fine-grained dioritic enclaves are abundant in these rocks. Their size is variable with average lengths of 10-30 cm.
Enclaves range in composition from gabbro dared, dared-tonalite, granodiorite in host granodiorites and dacitic enclaves in rhyodacites (Fig. 2). Granodiorite composition could result from potassic contamination by the host granodiorite.
Gabbro dioritic enclaves: These enclaves are concentrated in the Astaneh granitoid. They range from 5 to 100 cm in diameter, most common size with 20-70 cm (Fig. 3a).
These rocks have fine grain texture (Fig. 3b, c). The main minerals are plagioclase, (30-45vol.%), usually euhedral, more or less with variable degrees of sericitisation, shows zoning and the first felsic mineral to crystallize. They generally show SiO2 (51.72-57.5), Al2O3 (25.2-29.55), Na2O (5.23-8.66), CaO (5.4-11.96) and K2O (<1.0) contents. K-feldspar (<5 vol. %) forms anhedral to subhedral crystals, includes microcline-perthites and a late interstitial phase. They are usually altered to clay minerals. Quartz (<10 vol. %) forms anhedral crystals or aggregates with irregular disordered boundaries, normally occupies the interstices between plagioclas, display recrystallized features with adulatory extinction indicative of incipient solid-state deformation. Amphibole (25-35 vol.%), shows a euhedral prismatic habit, green colour and it is often twinned, associated with biotite and altered to biotite, chlorite, epidote and prehnite. The amphiboles are mainly magnesio-hornblende with a few actinolitic hornblendes on the (Si, P. f.u) Versus (Mg/Mg + Fe2+).
| Table 2: | Representative electron microprobe analysis of pargasitic
amphibole in the dacitic enclave and magnesio-hornblende in the
mafic microgranular enclave (MME, gabbro dared) from Astaneh pluton,
(Number of ions on the basis of 23 oxygen) |
| |
|
Amphiboles show (CaB> 1.5(1.7-1.85), (Na + K) A < 0.5 (0.2-0.3) and thus are calcic amphiboles occurring to Leake et al. (1997) classification, (Fig. 3d, 4a, Table 2) that usually in calc-alkaline granitoids. Biotite, the most abundant mafic mineral (5-15vol. %) appears in brown flakes. It is frequently associated with amphibole which crystallized before biotite. Most biotite has altered to chlorite, biotite is highly aluminous (Al/Al + Si + Mg + Fe = 0.2-0.22) and ferrous (Fe/Fe + Mg = 0.5-1), thus are biotite occurring to Foster (1960) (Fig. 4c, Table 3). Apatite and the less abundant zircon and allanite occur in all samples. These minerals account for 0.5-1.5% in the modal analysis.
Dioritic enclave: Dioritic enclaves in Astaneh with the dominant size of less than 50 cm are observed in MME. It is supposed that the small enclave within the large MME might be formed by multiple injection of mafic magma (Fig. 5a). Anyway, this more mafic small enclave might be suggesting that magma mixing/mingling was the most possible process to form more mafic small enclaves (Kim et al., 2002).
These rocks have porphyritic and pokilitic texture with plagioclase megacrysts and composed predominantly of plagioclase (40-50 vol. %), amphibole (5-10 vol. %), biotite (15-20 vol. %), alkali feldspar (<5 vol. %), quartz (<10 vol. %). Plagioclase is anhedral to subhedral plates, zoned and altered to sericite, epidote and calcite. Biotite occurs as brown kinking flakes, deformed and altered to chlorite, or broken down to sphene, prehnite, muscovite, opaque and quartz. It is frequently associated with amphibole which crystallized before biotite. Amphibole shows a euhedral prismatic habit, green colour and it is often twinned, associated with biotite and altered to biotite, chlorite, epidote and prehnite. Quartz crystals occur as anhedral to subhedral with adulatory extinction and a late interstitial phase. Alkali feldspar is anhedral to subhedral crystals. Zircon, sphene, apatite are conspicuous accessory minerals. Minor alteration products are sericite, chlorite, epidote, prehnite and calcite.
| Table 3: | Representative electron microprobe analysis of biotite in dacitic enclave and gabbro dioritic enclave, from Astaneh pluton (Number of ions on the basis of 11 oxygen |
|
|
|
| Fig. 3: | (a) MME in the Astaneh granitoids, (b) fine grain
texture micro gabbro dared, backscattered electron image (BSE) to
representative samples, (c) Bt, Amph and Plag in MME and (d) Euhedral
Magnesio-hornblende in mafic Microgranular enclave. Bt = biotite;
Amph = amphibole; Mineral abbreviations are according to Kretz (1983) |
| Fig. 4: | Classification of amphibole and biotite in gabbro
dioritic and dacitic enclaves in Astaneh pluton according to Leake et al. (1997) and Foster (1960), respectively. (a) showing
crystallization of magnesio-hornblende in gabbro dioritic dioritic,
(b) pargasite in dacitic enclave and (c): Plots of (a) R2+ (Fe2+ +Mn)-Mg-R3+ (Fe3+ +Ti +Al VI) of Foster (1960) |
| Fig. 5: | (a) Double enclave (enclave within enclave) in Astaneh
pluton, (b) poikilitic texture in dioritic enclave, (c) quartz ocelli
in dioritic enclave and (d) acicular apatite in Alkali feldspar |
The crystal shapes of K-feldspar, simple twinning and its zoned euhedral plagioclase inclusions found in the dioritic-tonalitic enclaves suggest that they have an igneous origin (Vernon and Paterson, 2002). Their mineralogy and texture show that have a multistage crystallization (Castro et al., 1991). These enclaves have a co-magmatic composition with their host. Accessory minerals include opaque phases, titanite and apatite. Chlorite, epidote and sericite occur as alteration phases. Dioritic enclave (e.g., E-29 and E-28) are essentially composed of calcic amphibole (magnesio-hornblende) and andesine plagioclase (An = 35-40) and subordinate biotite. Needle-like apatite preferentially occurs in plagioclase and quartz. Secondary mineral usually fill cleavages and intergranular boundaries.
Poikilitic texture which is commonly observed in the quartz dioritic enclave (Fig. 5b), large (4-5 mm in size) quartz or alkali feldspar phenocrysts including small (0.1-0.5 mm in size) plagioclase, amphibole, biotite and apatite.
| Fig. 6: | Harker variation diagrams of selected major element oxides |
This texture probably result from: (1) the growth of quartz and alkali feldspar phenocrysts during the crystallization, capturing pre-existing plagioclase, amphibole, biotite and so in common crystallization history and (2) late stage crystallization of felsic melt after the crystallization of abundant quench-generated plagioclase, amphibole, biotite and apatite in relation to magma mixing (Hibbard, 1991). If the poikilitic texture had been formed by the former case, the included minerals would have arranged after the crystal growth face of the quartz and alkali feldspar phenocrysts during the crystallization, however most of the included minerals orient randomly, therefore it is considered that the texture might be formed by the later case and partially dissolved portions are observed occasionally in the boundary between the phenocrysts and the included minerals, therefore by mafic magma injection into felsic magma, quartz and alkali feldspar phenocrysts were corroded and plagioclase, amphibole, biotite and apatite from mafic magma were supercooled during the crystallization of these phenocrysts in granitic magma.
The ocellar and ovoid textures (Fig. 5c) are found in MME. The ocellar texture is mantling quartz and alkali feldspar ovoid phenocrysts (1-2 mm in size) with fine-grained mineral association of amphibole±biotite±plusmn;plagioclase or a single mineral. Some ovoid occur without mantling. The ovoid grains of quartz and alkali feldspar were formed by the dissolution of pre-existing crystals owing to high temperature environment by the inflow of phenocrysts from the granite into the MME (Hibbard, 1991). This suggests that at the time of magma mafic injected, there were already existing alkali feldspar and quartz grains in the granitic system because compositions of MME could not crystallize quartz phenocryst in the early stage of crystallization (Johannes and Holtz, 1996).
MME have acicular crystals of apatite (Fig. 5d). Rapid growth of apatite in a quenched magmas results in acicular shape, rather than stubby prismatic or prismatic one (Reid et al., 1983). Acicular apatite in granitic rocks has also been identified as a mixing/mingling texture (Didier, 1987).
Dacitic enclaves: In these rocks, Quartz, plagioclase and biotite occurs as phenocrysts in a seriate texture. Normal alteration is to aggregates of chlorite, opaque minerals and epidote. These rocks contains numerous phenocrysts of pargasitic amphibole to CaB> 1.5(1.71- 1.77), (Na + K) A > 0.5(0.55-0.7) (Fig. 4b) and plagioclase in fine grain matrix of biotite and plagioclase.
The absence of clinopyroxene in gabbro dioritic, dioritic and dacitic enclave may result from rapid cooling of the mafic magma down to the stability field of amphibole and plagioclase, inhibiting nucleation of higher-temperature phases (Blundy and Spark, 1992).
Geochemical characteristics of the enclaves: Representative whole- rock major and trace elements compositions of Astaneh enclaves and average of host rocks are reported in Table 1. Major elements variations are shown in Harker diagrams in Fig. 6. The investigated rocks show compositional variations, especially in their silica content. The samples exhibit a range in SiO2 content from approximately 52 to 65wt%. Fe2O3, MgO, CaO, TiO2 and MnO abundances decrease with increasing SiO2, whereas K2O, Al2O3 and Na2O increase.
| Fig. 7: | Harker variation diagrams of selected trace element versus SiO2 content |
The highest MgO content in the plot is for gabbroic enclaves which have high modal proportions of Ca-amphibole.
The highest MgO content in the plot is for gabbroic enclaves which have high modal proportions of Ca-amphibole.
The trace elements behave in a similar way when plotted versus increasing SiO2 content (Fig. 7), decreasing trends for V, Cr, Ni, Co and increasing trends for Rb and Sr. Transition elements (Ni, Cr, Co, V) decrease with increasing SiO2 content, in agreement with their incorporation into early-crystallized ferro-magnesian silicates.
DISCUSSION
The Astaneh enclaves have gabbro dared to dioritic compositions. Enclaves with a magmatic texture are found in almost all the granitoid plutons. Vernon et al. (1988) have emphasized that rounded to ellipsoidal shapes to enclaves are strong indication of magma mingling and flow.
In felsic plutons, the volume of more mafic magma is much less than that of the more felsic magma, so that quenching and pillowing of the more mafic magma occurs; i.e., the magmas commonly mingling, rather than mixing (Kouchi and Sunagawa, 1983; Vernon, 1983, 1984; Sparks and Marshall, 1986).
These enclaves are microgranular in texture and rich in mafic minerals and, in descriptive terms, appear to be mafic microgranular enclaves (Didier and Barbarin, 1991) which are generally accepted to represent products of incomplete mixing (mingling) between mafic (enclave) and felsic (host granitoids) magmas (Eberz and Nicholls, 1988; Dorais et al., 1990; Didier and Barbarin, 1991). Indeed very fine-grained equigranular texture of MME (Fig. 3b) reflects that mafic magma was in near liquids state when it was injected into granitic magma.
The characteristic petrographic features of MME are the presence of: (i) acicular apatites (resulting from rapid growth in an internally quenching mixing system); (ii) poikilitic feldspars (containing mafic mineral inclusions); and (iii) quartz ocelli (resulting from the undercooling-generated mafic crystals of a mafic system around a nucleus of early-formed quartz minerals of felsic system), which are widely accepted as evidence of mixing/ mingling (Eberz and Nicholls, 1988; Hibbard, 1991; Vernon, 1991).
Gabbro- dioritic, dioritic and dacitic enclaves have I-type mineral assemblages that are broadly similar to those in the host granitoids except for the greater abundance of mafic minerals, such as amphibole.
Their chemistry contains the lowest values of SiO2 and the highest CaO and MgO for the enclaves. This similarity in mineralogy between enclaves and host rock is explained by Van Der Laan and Wyllie (1993) from a series of acid-basic experiments. According to authors, if mafic minerals are in equilibrium with the melt in the mafic domain and if the melts in both are mutually equilibrated, then the minerals in enclaves should also be in equilibrium with the melt in the granitic domain and therefore they should be similar to the mafic minerals simultaneously or subsequently precipitated from the granitic host magma with decreasing temperature. Moreover the same mineral assemblage would support thermal equilibrium states in granite and enclaves (Pitcher, 1991).
| Fig. 8: | REE and trace elements composition of enclaves normalised against their host granitoids |
This together with the occurrence of enclave within enclave (Fig. 5a) implies than mingling between these enclaves and host granodiorites.
The elevated Sr values displayed in these rocks may be attributed to the influence of the mafic magma (Fig. 8). We consider that the more Sr enrichmentin enclaves than host grandiorities ascribed toxenocrysts of oscillatory-zoned, calcic rich, plagioclase feldspars which are a known repository of Sr (Poli and Tommasini, 1991) and which were introduced by the injection of the mafic magma. Besides plagioclase, Sr may have also have been contributed through the presence of the calcic amphiboles, sphene and apatite.
According to Nardi and Lima (2000), the least hybridized enclaves are those with SiO2 lower than 55 wt.% and MgO higher than 4.5 wt.%, the slightly hybridized are those with SiO2 close to 56 wt.% , the moderately hybridized are those with SiO2 close to 58 wt.% and finally, the strongly hybridized enclaves are those with SiO2 varying from 60-65 wt.% . In Astaneh pluton, the least and slightly enclaves are restricted to the gabbroic and dioritic enclaves. In order to illustrate the geochemical effects of hybridism in the enclaves of the Astaneh granitoids, their major and trace components were normalised against the host rock average (Fig. 8). Rare earth element patterns of gabbro dared and dioritic enclaves normalised against the host rock average reproduce the flat shape except Eu contents controlled by added feldspar, as indicated by pronounced positive and negative Eu anomalies. So, the increase of Nb in enclaves of I-type granites can be interpreted as an evidence of preferential Nb diffusion to mafic microgranular enclaves (Green, 1995).
CONCLUSIONS
The enclaves in Astaneh granitoid are classified into microgabbrodiorite, microdiorite and less than microgranodiorite. Granodiorite composition could result from potassic contamination by the host granodiorite. These enclaves demonstrate that the clearest evidence for an origin through mingling processes is provided by field and petrographic features, such as chilled margins against the host granite, the presence of ocelli quartz and feldspar as megacrysts in dioritic enclaves, poikilitic texture, acicular apatites and double enclaves. In addition, geochemical normalised diagrams against average granite showed the dominant process in forming of enclaves was magma mingling.
ACKNOWLEDGMENTS
This study is part of the Ph.D Thesis by Z.T. Dr. Mohammad Ali Mackizadeh at University of Isfahan and Mehraj Aghazadeh at University of Tarbiat Moddares is thanked. The geochemical study was carried out in the University of Huelva (Spain) during a study leave of Z.T.