Geothermobarometry of Amphiboles in Intermediate to Basic Rocks from the Almogholagh Pluton in Western Iran

Journal of Mineralogical and Petrological Sciences, Volume 111, page 337–350, 2016

Geothermobarometry of amphiboles in intermediate to basic rocks from the Almogholagh pluton in western Iran

Manuchehr AMIRI, Ahmad AHMADI KHALAJI, Zahra TAHMASBI, Reza ZAREI SAHAMIEH and Hassan ZAMANIAN

Department of Geology, Faculty of Science, Lorestan University, Khoramabad, Iran

In this study, some barometries and thermometries were used to calculate the crystallization temperature and pressure of amphiboles in intermediate to basic rocks from the Almogholagh pluton in western Iran. Intersection among geothermometers was introduced as a method to estimate crystallization temperature, and the results obtained were compared with those of other methods. Results showed that obtaining the average pressure from the intersections between geothermometers and geobarometers is a suitable approach to determine the crystallization pressure of the analyzed points of amphiboles, whereas the crystallization temperature can be appro- priately determined from the intersection among geothermometers. In addition, the average temperature of the HB294 geothermometer is approximately equal to the average temperature in intersection among geothermometers. HB294 is apparently a suitable geothermometer to determine the crystallization temperature in intermediate to basic rocks. In this study, the average crystallization temperature and pressure using intersection methods were 714–719 °C and 6.5–7.5 kbar, respectively. Adopting these methods to the Almogholagh pluton, intermediate to basic rocks intruded at a depth of 24–28 km of the crust and at 674–759 °C.

Keywords: Almogholagh pluton, Amphibole, Geobarometry, Geothermobarometry, Geothermometry

INTRODUCTION the last two decades, Al geobarometry in amphiboles has been widely used to calculate the crystallization pressure Amphiboles are chemically and structurally complex min- of magma and to determine the emplacement depth of erals. According to Leake (1978), with the approval of the batholiths in the earth’s crust (e.g., Ague and Brandon, International Mineralogical Association (IMA), the sub- 1992; Anderson and Smith, 1995). Geobarometers are committee on amphiboles has presented the classification generally divided into two groups: empirical and experi- of amphiboles (Leake et al., 1997, 2004). This classifica- mental. Empirical geobarometers are based on calcic am- tion is important for geothermobarometric studies. Some phiboles in calc–alkalic masses. This type of geobarome- amphiboles, such as actinolite, are created through sub- ters includes that of Hammarstrom and Zen (1986) and solidus and secondary processes (Leake, 1971; Helmy et Hollister et al. (1987), referred to as Formulas 1 (Ham al., 2004) and thus are not used in geothermobarometric in Table 6) and 2 (Hol) in the APPENDIXES. Experimen- studies. Actinolite is identified from other amphiboles on tal geobarometers, such as those developed by Johnson the basis of diagrams in the above–mentioned amphibole and Rutherford (1989) and Schmidt (1992), are shown classification. as Formulas 3 (Joh) and 4 (Sch) in the APPENDIXES. Amphibole composition varies depending on the The pressures in the Formula of Schmidt (1992) are sim- bulk composition of magma, temperature, oxygen fugac- ilar to and thus confirm those achieved from the contact ity, and pressure during crystallization. Al content in am- aureole of igneous masses (Ague, 1997). Using the data of phibole is affected by the four aforementioned factors. In- Schmidt (1992), Anderson and Smith (1995) designed a creasing temperature and pressure increase Al2O3 content geobarometer [shown as Formula 5 (And) in the APPEN- in amphiboles, and their effects are considerably greater DIXES] that considers the effect of temperature. than those of the two other factors (Moody et al., 1983). In Hornblende–plagioclase thermometry is the most commonly used method for calc–alkaline igneous rocks doi:10.2465/jmps.151228 (Stein and Dietl, 2001; Ernst, 2002). Adopting an ideal M. Amiri, [email protected] Correspnding author structure for amphiboles and a non–ideal structure for pla- 338 M. Amiri, A.A. Khalaji, Z. Tahmasbi, R.Z. Sahamieh and H. Zamanian gioclases, Blundy and Holland (1990) developed a thermometer [shown as Formula 6 (BH90) in the APPEN- DIXES] that considers the effect of pressure. The ideal amphibole model has been criticized for its oversimplicity (e.g., Poli and Schmidt, 1992). In addition, temperatures were obtained for aluminous hornblendes coexisting with plagioclase, whereas erroneously high temperatures were obtained for garnet–amphibolites (Mengel and Rivers, 1991). Holland and Blundy (1994) found that within– and across–site interactions in amphiboles vary. There- fore, assuming a non–ideal amphibole, Holland and Blun- dy (1994) presented two geothermometers. One, which is presented as Formula 7 (HB194) in the APPENDIXES, can be used for amphiboles coexisting with plagioclases Figure 1. Geotectonic map showing the spatial distribution of Ira- in quartz–bearing rocks. The other, which is presented as nian southwestern tectonic units and major plutonic bodies, as Formula 8 (HB294) in the APPENDIXES, can be used for well as the location of study area (square), in the Sanandaj–Sir- amphiboles in rocks with or without quartz. jan Zone according to Stocklin and Nabavi (1972) and was The present study aims to estimate the crystalliza- modified by Sepahi et al. (2009). Abbreviations: SQ, Saqqez; tion temperature and pressure of amphiboles in inter- SD, Sanandaj; GV, Ghorveh; AM (square as Fig. 2), Almogho- lagh; HD, Hamedan (Alvand); AR, Arak; AS, Astaneh; BJ, Bor- mediate to basic rocks from the Almogholagh pluton oujerd; AG, Aligudarz; AZ, Azna; MT, Muteh; GP, Golpaygan; by using some of the aforementioned methods. It also KG, Kolah–Ghazi; SJ, Sirjan; and SK, Siah Kouh. Color version introduces intersection among geothermometers as a of Figures 1 is available online from http://doi.org/10.2465/ new method to estimate crystallization temperature and jmps.151228. compares the results of this method with those of other methods. Jurassic (Berberian and Berberian, 1981). This phenome- GEOLOGICAL SETTING non deformed rocks and some intrusive bodies in S–SSN (Late Triassic) and emplaced numerous intrusive bodies Stocklin and Nabavi (1972) divided the Iranian plateau ranging from gabbro to granite in N–SSZ (Fig. 1). The into eight segments: Zagros folding, Zagros thrusting, the ages of zircon U–Pb imply that many SSZ granitoids Sanandaj–Sirjan Zone (SSZ), the Urumieh–Dokhtar mag- were emplaced during the Middle Jurassic (e.g., Shahbazi matic arc (UDMA), central Iran, Alborz, Kopeh Dagh, et al., 2010), Middle to Late Cretaceous (e.g., Ghalam- and eastern Iran. The southwestern part of Iran was div- ghash et al., 2009), and Early Eocene (Mazhari et al., ided into three parallel NW–SE trending tectonic zones 2009). Closure of the oceanic crust domain was marked (Alavi, 1994), namely, the Zagros Thrust and Fold Belt, by the obduction of ophiolites along the main Zagros the SSZ, and the Tertiary–Quaternary UDMA (Fig. 1), thrust in the Late Cretaceous to Paleocene (Agard et which is an integral part of the Alpine–Himalayan oro- al., 2005) and of some gabbroic bodies in the Upper Eo- genic system that resulted from Arabian–Central Iranian cene to Oligo–Miocene, indicating that the final closure microplate continental collision after the subduction of of the Neo–Tethys and collision of Arabian plate and the Neo–Tethys oceanic crust beneath Central Iran (Sen- Central Iran occurred during the Neogene period (Berber- gör, 1984). The SSZ is geographically subdivided into ian and Berberian, 1981; Azizi and Moinevaziri, 2009). southern (S–SSZ) and northern (N–SSZ) parts by Efte- The Almogholagh pluton forms part of a linear belt kharnejad (1981) (Fig. 1). During the Paleozoic period, of N–SSZ intrusions in western Iran (Fig. 1). Country the SSZ was part of northeast Gondwanaland, separated rocks, namely, the Hamedan phylites, comprise the Tri- from the Eurasian plate by the Paleo–Tethys ocean (Go- assic–Jurassic metapelites (Mohajjel et al., 2003). Several lonka, 2004). During the Middle to Late Triassic, coeval regional and contact metamorphisms, including garnet– with the closure of the Palaeo–Tethys in the North, a rift- mica schists, interbedded metavolcanics, marbles, and ing episode along the Zagros belt resulted in the opening dolomitic limestones, occur around the pluton. Field ob- of a new ocean called Neo–Tethys, and the SSZ was sep- servations revealed that Almogholagh intrusive rocks can arated from the Arabian plate (Gondwanaland) as a part be classified into three groups, namely, dark grey diorite of Eurasia. Inception of subduction of Neo–Tethys was or gabbroic diorite, grey–colored quartz syenite, and inferred to have occurred during the Late Triassic–Early light–colored quartz monzonite. Other researchers have Geothermobarometry of amphiboles in the Almogholagh pluton, Iran 339 reported the presence of rocks, such as granite, quartz monzonite, monzonite, and monzodiorite, in this pluton (Shahbazi et al., 2015). Mohajjel and Izadi Kian (2007) have proven the occurrence of four deformations (D1–D4) in the Almogholagh area that the pluton intruded during D2 deformation. Valizadeh and Zarian (1976), Shahbazi et al. (2015), and Amiri (2015) found that the ages of the Almogholagh masses are 144 ± 17, 95–138, and 131 ± 25 Ma by using three–point Rb–Sr whole–rock isochron, U– Pb and U–Th–Pb methods (for magmatic sphenes), and seven–point Rb–Sr whole–rock isochron, respectively. Figure 2. Locational and lithological map of the Almogholagh SAMPLES AND METHODS plutonic rocks. Color version of Figures 2 is available online from http://doi.org/10.2465/jmps.151228. In this study, approximately 80 samples were collected from the Almogholagh pluton during a 20–day operation. A total of 85 thin sections of these samples were prepared and studied using an optical microscope. Representative samples (22 samples) were selected to analyze whole– rock geochemistry. Major and trace elements were analyzed through inductively coupled plasma–mass spec- trometry equipped with an LF–200 system at the ACME Analytical Laboratories in Vancouver, Canada, and the results are presented by Amiri (2015). Different lithological diagrams were then drawn, and rock types were identified using the percentages of major oxides and GCDkit software. In addition, 16 square polished 1 mm × 1 mm × 0.5 mm disks were prepared for scanning elecron micros- copy to study the mineral phases and to obtain back–scattered electron images. The samples were coated with a Figure 3. Backscattered electron images of minerals in (a), (b) layer of 20 nm–thick carbon by using the sputtering acidic and (c) intermediate rocks. (a) Photograph of AM20 con- method and then studied under a MIRA3 field–emission taining Ch and Cl secondary crystals. (b) Photograph of AM12 scanning electron microscope. containing Ac secondary amphibole. (c) Photograph of AM48 – Three thin polished sections of intermediate to basic containing Am and coexisting Pl. Abbreviations: Cl, Zr bearing clinochlore; Ch, Zr–Fe–bearing clinochlore; Ru, rutile; Am, am- rocks (Am48, Am47, and Am46) and acid rocks (Am23, phibole; Zir, zircon; Pl, plagioclase; Spn, sphene; Ilm, ilmenite; Am07, and Am07–2) were prepared for microprobe anal- Ac, actinolite; Ap, apatite; Or, orthoclase; Qz, quartz; Wo, tung- yses. The sampling site is shown in the lithological map sten–bearing mineral. Symbols: the circles indicate the analyzed (Fig. 2). We tried our best to obtain the less altered sam- points (by EPMA). Color version of Figures 3 is available on- ples. The amphiboles and coexisting plagioclases in the line from http://doi.org/10.2465/jmps.151228. sections were analyzed at the Iran Mineral Processing Research Center Laboratory by using Cameca SX 100 Electron Probe Micro–Analyzer, which was operated at green–brown amphiboles lying near each other). Analyt- a probe current of 20 nA with an accelerating voltage ical data on plagioclases were calculated as atomic values of 20 kV. The counting duration is usually 35 s for the of the elements based on eight atoms of oxygen and for peak of each element. The electron beam diameter was intermediate to basic rocks are presented in Table 2. The set to 10 µm. The rims of amphibole and plagioclase Fe2+ and Fe3+ contents of the analyzed points in amphib- crystals that are located close to one another were ana- oles were estimated by using the method described by lyzed (e.g., Fig. 3c). A total of 44 points were analyzed, Spear and Kimball (1984), and the structural formulas 20 of which belong to intermediate to basic rocks (Tables of the amphiboles were determined (Tables 1 and 3). 1 and 2) and 24 belong to acid rocks (9 points in green The amphiboles were also classified and named using amphiboles, 9 points in plagioclases coexisting with am- the structural formulas and the methods in IMA1997 phiboles, and 6 points in an association of euhedral (Leake et al., 1997) and IMA2004 (Leake et al., 2004). 340 M. Amiri, A.A. Khalaji, Z. Tahmasbi, R.Z. Sahamieh and H. Zamanian

Table 1. Results of the microprobe analysis and the calculated atomic values of the amphiboles

Upon ensuring that the analyzed points are not subsolidus grained quartz in the field (~ 3–10%), rhombic sphenes as amphiboles (actinolite), we calculated the crystallization accessory minerals (~ 2%), and rhombic magnetites and pressure and temperature by using some of the aforemen- ilmenites as conspicuous opaque minerals (~ 4%). Clino- tioned methods. The results obtained by using these pyroxene is rarely observed in some samples. The plagio- methods were compared. Finally, geothermometer inter- clases occur as euhedral to subhedral without zoning section was used as a method to estimate the temperature crystals, although they display polysynthetic twinning of magma during the crystallization of the analyzed am- and generally an extinction angle of lower than 35°. phiboles, and the results obtained by using this method The plagioclases are mainly of oligoclase or andesine were compared with those of existing methods. types, and some of them appear as albite or labradorite. On the basis of the diagram prepared by Middlemost RESULTS AND DISCUSSIONS (1994), the Am–46, Am–47, and Am–48 samples are identified either as quartz diorite or monzodiorite rocks. Petrography The second group (acid rocks) includes light– or grey–colored porphyritic rocks that contain feldspars On the basis of the results of the observation of the thin and amphiboles as their coarse minerals. This group con- sections, intrusive rocks were classified into two groups. tains amphiboles (~ 15–20%), alkalic feldspars and pla- The first group is labeled as gabbroic diorite and diorite gioclases (~ 50–55%), leucoxene + ilmenite + sphene + in the lithological map (Gd in Fig. 2); this group includes magnetite (~ 10%), and fine–grained quartz (~ 15–25%). dark, granular, and medium to fairly coarse grained rocks Shorl–F tourmaline is rarely observed in some thin sec- that contain light green to olive green amphiboles (~ 30– tions. Plagioclases in acidic rocks with an extinction an- 35%), alkalic feldspar and plagioclase (~ 55–60%), fine– gle between 12 and 20° correspond to albite or oligoclase Geothermobarometry of amphiboles in the Almogholagh pluton, Iran 341

Table 2. Results of the microprobe analysis and the calculated atomic values of the plagioclases

type. Two types of amphiboles are observed. One is a Geochemistry of pluton green–brown euhedral early–formed amphibole, and the other is a green subhedral late–formed amphibole. Acid In the diagram prepared by Cox et al. (1979), Almogho- rocks are divided into two sub–groups, namely, light–grey lagh intrusive rocks are located beside the separating bor- quartz monzonite (Lg in Fig. 2) and dark–grey quartz der of the alkaline series from the subalkaline and tho- syenite (Dg in Fig. 2). The quartz syenite has finer grains, leiitic series. In the diagram of Irvine and Baragar (1971), and their amphiboles and opaque minerals (sphenes and the intermediate to basic rocks are placed in the calc–al- magnetites) are scattered, whereas the amphiboles and kaline series. The results of a systematic geochemical opaque minerals in the quartz monzonite exhibit clots study based on various diagrams show that the intermedi- (microglomeroporphyritic texture). The granophyric tex- ate to basic rocks are calc–alkaline I–type intrusions and ture of quartz monzonite suggests that the quartz was associated with late or post–collision stage (i.e., III–type subjected to a sudden decrease in pressure and probably of Harris et al., 1986). In addition, the acid rocks are A2– could have come near the surface during the last stage of type intrusions and associated with late or post–collision formation. Secondary crystals such as actinolite, clino- stage. In Ce and Nb versus 10000Ga/Al diagrams of chlore, and leucoxene formed in acidic rocks. These min- Whalen et al. (1987), all quartz monzonite and quartz erals show that active fluids attacked amphiboles and pla- syenite samples (acid rocks) plot in the A–type granitoids gioclases in acidic rocks (Figs. 3a and 3b). On the basis field, whereas Almogholagh intermediate to basic (gab- of the diagram prepared by Middlemost (1994), the Am– broic diorite) samples plot in the I– and S–type granitoids 23, Am–07, and Am–07–2 samples are identified either as field. The Almogholagh intrusions exhibit the following quartz monzonite or quartz syenite. characteristics: εSr > 0, εNd > 0, Ba/La > 3, (Rb/Ba)N > In some petrologic diagrams (Cox et al., 1979; De La 1; distinctive spiked pattern with peaks at Rb, Ba, Th, K, Roche et al., 1980; Debon and Lefort, 1983; Middlemost, Sr, and Zr; light–REE enriched pattern; low abundance of 1994), the following various rocks are identified as fol- elements with high ionic potential (e.g., Dy, Y, Yb, Lu, lows: quartz–syenite, quartz–monzonite, granite, quartz– and Sm); and high aboundance of Nb, with weak nega- diorite, diorite, gabbroic diorite, and gabbro, which are tive anomalies of it (Fig. 4). These properties are appa- generally divided into two groups as mentioned above. rently a characteristic of subduction–related magmas, demonstrating the involvement of subduction zone fluids, 342 M. Amiri, A.A. Khalaji, Z. Tahmasbi, R.Z. Sahamieh and H. Zamanian

Table 3. Names and structural formulas of the analyzed points (in amphiboles)

high ﬂux of mantle–derived halogen–rich volatiles, and centration of Zr in both acidic and intermediate to basic contamination within the crust during the petrogenesis rocks is considerably high, 589 and 165 ppm, respective- of Almogholagh intrusions (Amiri, 2015). ly, and shows a distinctive spiked peak in spider diagrams The acid rocks generally contain high concentra- (Fig. 4). When large quantities of ﬂuorine and/or chlorine tions of Na2O+K2O (9.48%), Sn (6.9 ppm), Ga (22.7 exist in the environment where magma is produced and ppm), Ce (8.5 ppm), and Y (57 ppm). The average con- crystallized (e.g., the presence of Shorl–F tourmaline in Geothermobarometry of amphiboles in the Almogholagh pluton, Iran 343

magma resulting from the melting of the parent rock, and Zr increases substantially during fractional crystallization. Harris and Marriner (1980) used the high ﬂux of mantle–derived halogen–rich volatiles to induce melting and to provide a high concentration of alkalis and high- ﬁeld–strength elements.

Chemistry of amphiboles

The microprobe can not distinguish between Fe2+ and Fe3+. Consequently, analyses are usually in terms of total iron, recorded as FeO. However, Fe2+ and Fe3+ occupy diﬀerent types of cation sites in amphiboles, and the 3+ Figure 4. Primitive mantle–normalized element distributions for amount of Fe in an amphibole is critical in determining the intrusive rocks obtained from the Almogholagh pluton the A–site occupancy (Spear and Kimball, 1984). There- (the primitive mantle factors are based from those reported by fore, the amounts of Fe2+ and Fe3+ were calculated by McDonough and Sun, 1995). Color version of Figures 4 is using a method employed in IMA to determine the struc- available online from http://doi.org/10.2465/jmps.151228. tural formula of amphiboles. The aforementioned method was used to determine the structural formulas of the an- Almogholag pluton), the F and C1 in the magma distort alyzed points (Table 3). To identify and determine the the aluminosilicate framework, providing binding sites amphibole type in accordance with the instructions of for the highly charged cations and stabilize the com- IMA, we established diagram 5a on which the analyzed plexes of large, highly charged metal ions (e.g., Zr). An points were subsequently plotted. All of the samples were excess of alkalis over alumina and high F in the melt placed in the calcic amphibole group. Many researchers inhibits zircon formation and promotes the formation of suggested that the presence of calcic amphiboles in gran- zircono–ﬂuoride melt complexes (Whalen et al. 1987). itoid rocks indicates that these rocks belong to I–type This phenomenon leads to the enrichment of Zr in the magma (e.g., Chappell and White, 1974). In addition,

Figure 5. Plots of the analyzed amphiboles obtained from the Almog- holagh plutonic rocks on classification diagrams based on the criteria proposed by Leake et al. (1997, 2004). (a) Amphibole classification diagram based on five general cat- egories. (b) Classification diagram of calcic amphiboles with Na + K of more than 0.5 apfu and Ti of less than 0.5 apfu. (c) Classification diagram of calcic amphiboles with Na + K of less than 0.5 apfu and Ca of less than 0.5 apfu in their A site. Symbples: Circles indicate the amphiboles in intermediate to basic rocks, whereas squares indicate the amphiboles in acidic rocks. Color version of Figures 5 is available online from http://doi.org/10.2465/ jmps.151228. 344 M. Amiri, A.A. Khalaji, Z. Tahmasbi, R.Z. Sahamieh and H. Zamanian

Table 4. Obtained pressures for the analyzed points (in amphiboles)

the diagrams showed that the amphiboles in the inter- continental crust (Tulloch and Challis, 2000) and a cal- mediate and basic rocks belong to either the hastingsite culated error factor of ±0.5 kbar for the pressure, we or magnesiohastingsite subgroups (Fig. 5b), which con- found that the analyzed points crystallized at a depth of tains less than 6.4 (Si) and 2 (Mg) apfu. The amphiboles 24–28 km. in the acidic rocks belong either to the ferrotschermakite subgroup, which contains less than 5.8 (Si) and 0.5 (Mg) Geothermobarometry of amphiboles apfu, or to the actinolite subgroup (Fig. 5c), which contains more than 7.6 (Si) and 2.7 (Mg) apfu. According to To determine the formation temperature of the amphib- Leake (1978), magmatic amphiboles contain less than 7.3 oles, we first estimated the Fe2+ and Fe3+ contents in ac- apfu of Si atoms, and amphiboles containing more than cordance with the method employed by Holland and 7.3 apfu of Si atoms are derived via the subsolidus pro- Blundy (1994) and determined the structural formulas of cess (Agemar et al., 1999). The amphiboles in the acid the amphiboles. The aforementioned method is similar to rocks from the Almogholagh pluton are not suitable for but different to a certain extent from the methods em- geothermobarometry because the ferrotschermakites do ployed by Spear and Kimball (1984) and Robinson et not coexist with plagioclases (they are probably derived al. (1982). On the basis of the calculated pressures pres- from magma that initially crystallized at a considerable ented in Table 4, the temperatures were calculated by the depth), and the actinolites were derived through the sub- BH90, HB194 and HB294 geothermometers, and results solidus process. None of the amphiboles in the intermedi- are presented in Table 5. The temperatures that were esti- ate to basic rocks were formed through the subsolidus mated at the pressure derived from the formula established process. Thus, all of them are suitable and selected for by Schmidt (1992) were selected as initial temperatures. geothermobarometry. Subsequently, the values for pressure obtained by using the method of Anderson and Smith (1995) were based Geobarometry of amphiboles from the initial temperatures and then placed for iteration in the spreadsheet program (prepared by Lawford Ander- Using the Altotal value obtained from the structural for- son, University of Southern California). Finally, the tem- mula of amphiboles (Table 3) and the four formulas of peratures of HB194, HB294, and BH90 were calculated. empirical and experimental geobarometers (formulas 1, The values for pressure obtained using the formula pro- 2, 3, and 4 in the APPENDIXES), we calculated the crys- posed by Anderson and Smith (1995) were subsequently tallization pressure of the analyzed points (Table 4). The calculated on the basis of the HB294 temperature. Accord- pressure computed by using the Al in amphiboles shows ing to Anderson (1996), the temperature of HB294 is more the crystallization depth of the analyzed points in the am- appropriate to compute for the pressure than the other phiboles and may reflect the depth at which hornblendes temperatures. crystallize rather than the pressure at which granitoid rocks consolidate, that is, upward movement may contin- Intersection of geobarometers with geothermometers ue following amphibole crystallization (Ghent et al., 1991). The average pressure of the analyzed points is ~7 Sayari (2011) stated that the temperature fluctuates within kbar. Using the conversion factor 1 kbar = 3.7 km for the a certain range and the calculated value does not display Geothermobarometry of amphiboles in the Almogholagh pluton, Iran 345

Table 5. Geothermobarometric results for the analyzed points (in amphiboles)

the desired precision when pressure is calculated using a diagram. In other words, geothermometers are highly geobarometer and its value is used in a geothermometer. sensitive to temperature variations but are hardly depend- When the equations are drawn as graphs, the geothermo- ent on pressure. By contrast, geobarometers are charac- metric curves display low values for the changes in pres- terized by high dP/dT values as revealed when they are sure (dP)/changes in temperature (dT) ratio in the P–T drawn on a P–T diagram, indicating that they are consid- 346 M. Amiri, A.A. Khalaji, Z. Tahmasbi, R.Z. Sahamieh and H. Zamanian

Table 6. Results of the intersections between geobarometers and geothermometers

Ham, formula 1; Hol, formula 2; Joh, formula 3; Sch, formula 4; And, formula 5; BH90, fomula 6; HB194, formula 7; HB294, formula 8 (in the APPENDIXES)

erably more sensitive to pressure variations than to tem- Intersection of geothermometers perature variations. Therefore, these two aforementioned curves intersect at a point that more accurately represents As mentioned earlier, geobarometers and geothermome- the crystallization temperature and pressure. The three ters are sensitive to dP and dT, respectively. Therefore, geothermometers (i.e., BH90, HB194, and HB294) in the intersection of a geobarometer with a geothermometer the top row of Table 6 intersected with five geobarome- determines the appropriate pressure because both curves ters (i.e., Ham, Hol, Jon, Sch, and And) in the bottom consist of a pressure parameter and at least one of which row of Table 6, and the pressure and temperature from is sensitive to dP. However, the use of the intersection to their intersection were determined (Table 6). The average express the temperature may not be suitable because only pressure and temperature of the analyzed points in the one of two curves consists of a temperature parameter. By amphiboles are 7.0 kbar and 734 °C respectively, which contrast, in the intersection of two geothermometers, both are approximately equal to the average values (6.7 kbar curves consist of a temperature parameter and are sensi- and 742 °C) in Table 5. In addition, the pressure is equal tive to temperature changes; therefore, the average tem- to the average values (7.1 kbar) in Table 4. These differ- perature obtained from the intersection of geothermome- ences are not significant at a 95% confidence interval. ters with one another is closer to the real temperature, A relatively high correlation between temperature although the pressure resulting from their intersection is and pressure was observed in Table 5 (R2 = 0.99) and not accurate and is different from the real pressure be- Table 6 (R2 = 0.8). The equations showing the relationship cause none of the curves are sensitive to pressure. The between temperature and pressure show that the tem- intersection of BH90 and HB294 for point Am47–2is perature changes by ~ 5–10 °C when the pressure changes shown in Fig. 6 as an example. The three geothermom- by 1 kbar. eters, namely, BH90, HB194, and HB294, were intersected with one another for all analyzed points (with the ex- Geothermobarometry of amphiboles in the Almogholagh pluton, Iran 347

Figure 7. Correlation between the average temperature and the average pressure from the HB294 geothermometer in amphiboles (based on Table 5). Color version of Figures 7 is available online from http://doi.org/10.2465/jmps.151228. Figure 6. Intersection of BH90 and HB294 for AM47–2 point. Color version of Figures 6 is available online from http:// doi.org/10.2465/jmps.151228. kbar, the temperature change is equal to 2.5 °C. There- fore, the average crystallization temperature using the in- Table 7. Temperatures at the intersections of two pair geother- tersection of geothermometers of the analyzed points in mometers the amphiboles is 714–719 °C. According to the different equations of geothermometers (APPENDIXES), we assumed that the crystallization temperature changes than the average temperature is equivalent to ±40 °C. In sum- mary, the amphiboles in intermediate to basic rocks of the area crystallized at a depth of 24–28 km of the crust and at 674–759 °C. The aforementioned temperatures were confirmed by zircon saturation thermometry (TZr). TZr provides a simple and robust means to estimate the temperatures of magma (Watson, 1979). Zircon saturation temperatures calculated from bulk–rock compositions provide the minimum estimates of melt temperature if the magma was undersaturated with Zr and the maxima if the magma was saturated with Zr (Miller et al., 2003; Zhao et al., ception of HB194 with BH90), and the results of their 2012). The temperatures of the melt system were estimat- intersections are presented in Table 7. The average tem- ed by using Zr solubility in granitoid melts as proposed peratures obtained from the two intersecting pairs of geo- by Watson (1979) and Watson and Harrison (1983). Their thermometers are similar at a 95% confidence interval. experiments showed that the partition coefficient (DZr zir- Their average temperature (716 °C) differs from the tem- con) is a function of the parameter M =(Na + K +2Ca)/ perature in Table 5 (742 °C) by only ~ 26 °C, which is (Si × Al) and temperature. If a granitic melt is saturated not significant at a 95% confidence interval. with Zr, then its temperature may be calculated from the The average temperatures of HB294 given in Tables measured major element and Zr contents. The calculated 5 and 6 are 709 °C and 714 °C, respectively, and they are zircon saturation temperatures for Almogholagh inter- considerably close to the obtained average results of the mediate to basic rocks (I–type) range from 591 °C to 741 intersection of geothermometers (716 °C, Table 7). This °C with an average of 679 °C, which is significantly low- result demonstrates that this geothermometer (HB294) er than that of typical A–type rocks in the area. The pres- can provide a good estimate of the crystallization temper- ence of a significant amount of amphiboles (~ 30–35%) ature in the intermediate to basic rocks. A relatively high in intermediate to basic rocks from the Almogholagh plu- correlation exists between the temperature and pressure ton of calc–alkaline affinity (I–type magma) shows that that were obtained by the HB294 geothermometer (Table the hydrous conditions play an important role in magma 2 5, R = 0.999). The equation between temperature and formation. The presence of a significant amount of H2O pressure (Fig. 7) shows that the temperature changed in a liquid stabilizes amphiboles during the crystallization by ~ 5 °C against a pressure of 1 kbar. However, if the of magma and alters its physical properties (low viscos- calculated error factor for pressure is assumed to be ±0.5 ity, low density, and low liquidus/solidus) and phase re- 348 M. Amiri, A.A. Khalaji, Z. Tahmasbi, R.Z. Sahamieh and H. 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APPENDIXES

The Geobarometry and geothermometry equations were used for calculating the pressure or temperature and mentioned in the text, as equations 1 to 8 have been presented in the below. In all formulas, the temperature (T) is in centigrade degree (°C) and the pressure (P) is in kilobar (kbar).

1) P(±3kbar) = −3.92 + 5.03Altotal (Hammarstrom and Zen, 1986) 2) P(±1kbar) = −4.76 + 5.64Altotal (Hollister et al., 1987) 3) P(±1kbar) = −3.46 + 4.23Altotal (Johnson and Rutherford, 1989) 4) P(±0.6kbar) = −3.01 + 4.76Altotal (Schmidt, 1992)

(Anderson and Smith, 1995)

6) (Blundy and Holland, 1990)

(Holland and Blundy, 1994) X Pl Y ¼ 12:0ð1 X pl Þ2 3:0 In above formula, for ab > 0.5, then Yab = 0.0 otherwise ab ab KJ

(Holland and Blundy, 1994) X pl Y ¼ 12:0ð2:X pl 1Þþ3:0 In above formula, for ab > 0.5, then Yab–an = 3.0KJ otherwise aban ab KJ