Chapter IV MINERAL CHEMISTRY

Transcription

1 Chapter IV MINERAL CHEMISTRY

2 Chapter-IV MINERAL CHEMISTRY 4.1 INTRODUCTION In this chapter, chemical analyses of different minerals present in various rocks of Mashhad granitoid plutons have been presented. Chemical composition of different minerals like feldspars, biotite and amphibole in granitoids will help to understand the various igneous process during the formation of different types of granitic rocks. Composition of igneous biotites will reflect the magma composition, because experimental work has shown that the mineral biotite in granitic rocks will continuously equilibrate with the host liquids. Apart from biotites, other ferro-magnesian minerals also reflect the whole rock composition and consequently the origin of granitoids. Electron Microprobe Analyses (EPMA) of different minerals have been carried out for those rock samples for which detailed information on microtextures are available. During EPMA analyses, care has been taken in recording any chemical zonation that exist in different silicate minerals by analysing the core and rim compositions. Wherever the mineral show significant zoning pattern in their chemistry, chemical zonation of such minerals have been reported. The mineral analyses reported in this chapter represent the mean of three to five data points during EPMA analyses. While carrying out EPMA analyses, different points have selected in minerals, avoiding alteration zones. While selecting minerals for EPMA, emphasis is given to analyse the co-existing mineral pairs like plagioclase and K-feldspar, plagioclase and amphibole, and titanium in amphibole and biotite. This is to evaluate the cation-exchange like Ca, Na and Al between two feldspars (plagioclase and K-feldspar) and Mg, Fe and Al between plagioclase and amphiboles and Ti content in amphibole and biotite. This has been done in order to evaluate the temperature and depth of crystallization of granitoids in the area using different thermodynamic models.

3 4.2 MINERAL CHEMISTRY Plagioclase: EPMA analyses of plagioclase present in different rocks of Mashhad granitoids has been analysed and presented along with structural formula. As described in Chapter II, porphyritic plagioclase present in diorites and granodiorites from Mashhad area exhibit strong zonation. Chemical analyses were carried out from core to rim of porphyritic plagioclase in both diorite and granodiorite and the data is presented in Table 4.1 and 4.2. The SiO 2 content in plagioclase vary from to wt.% with Al 2 O 3 content varying from 25 to wt.%. Higher the value of SiO 2 in plagioclase, lower the value of Al 2 O 3 content, indicating negative correlation between these two chemical components. The amount of TiO 2 is negligible ( < 0.2 wt.%), with very low values of MnO ( < 0.04 wt.%), MgO ( < 0.03 wt.%) and Cr 2 O 3 (0.13 wt.%) component. Major variation is recorded mainly with respect to An and Ab component in plagioclase with very minor orthoclase component ( varying from 0.23 to 0.72 mole percent, Table 4.1 and 4.2). EPMA analyses of porphyritic, zoned plagioclase grains in diorite from core to rim is presented in Table 4.1. Plagioclase show significant zoning pattern with higher amount of CaO content ( An 61 ) indicating labradorite composition in the core to lower amount of CaO ( An 46 ) indicating andesine composition in the rim (Table 4.1). Chemical analyses of another grain of porphyritic plagioclase in granodiorite from Vakilabad show significant zoning pattern with core composition rich in anorthite component (An 58 ) indicating labradorite composition with rim composition poor in anorthite component (An 32 ), indicating andesine composition (Table 4.2). Plot of anorthite content in core and rim of plagioclase in diorite and granodiorite show presence of significant zoning pattern with increasing in albite component from core to rim with concomitant decrease in anorthite component from core to rim (Fig. 4.1 and 4.2).

4 EPMA analyses of different rocks from granodiorite, monzogranite, pink granite, tourmaline granite and leucogranite is presented in Table 4.3. They generally show higher amount of SiO 2 content varying from to wt.% with Al 2 O 3 content varying from to wt.% ( Table 4.3). The amount of TiO 2 ( < 0.01 wt.%), FeO (< 0.12 wt.%), MnO ( < 0.04 wt.%) with very low orthoclase component ( varying from 0.64 to 1.47 mole percent). There is a general decrease in anorthite in plagioclase from granodiorite to leucogranite with plagioclase of oligoclase in composition in granodiorite to mainly albite composition in tourmaline granites ( Table 4.3). K-feldspar: EPMA analyses of K-feldspars present in monzogranite, pink granite and tourmaline granites have presented in Table 4.5 along with structural formula. The SiO 2 content vary from to wt.% with Al 2 O 3 content varying from to wt.%. The amount of FeO ( < 0.31 wt.%), MnO ( < 0.06 wt.%), MgO ( < 0.79 wt.%) with very low anorthite component ( < 3.68 mole percent). There is a significant variation in orthoclase component with lower amount of orthoclase ( Or 81 ) in monzogranite to higher amount of orthoclase component in pink granite ( ) and tourmaline granite ( 97.19). Biotite: Biotite is the common mafic mineral occurring in all types of rocks in the Mashhad granitoid plutons. Biotite show varying pleochroism from light yellow to dark brown in diorites and leucogranites and light yellowish green to brownish green colour in monzogranites, indicating variation in chemical composition of biotite in different granitoids. EMPA analyses of biotites from different rock types along with structural formula is presented in Table 4.6. Biotite show variation in SiO 2 content from to wt.% with TiO 2 content varying from 1.51 to 3.23 wt.%. (Table 4.6). The Al 2 O 3 content vary from to wt.% which

5 show negative correlation with silica. The amount of Cr 2 O 3 ( < 0.41 wt.%), MnO ( < 0.97 wt.%) and Na 2 O content ( < 0.13 wt.%) is very low. There is a significant variation in the content of FeO and MgO in different rock types. The X Mg vary from 0.32 to 0.20 ( Table 4.6). Higher values of X Mg ( 0.32) is recorded in diorites with lower X Mg values of 0.20 to 0.23) in pink granites and leucogranites (Table 4.6). The composition of biotite fall in the phlogopite-annite field in ASF diagram (Fig: 4.3). Chemical analyses of biotites when plotted on FeO Vs MgO, MgO Vs Al 2 O 3, FeO Vs Al 2 O 3 plot mainly in the fields of peraluminous granites with few samples plotting in the fields of calc-alkaline granites ( Fig. 4.4, 4.5 and 4.6). However, in the ternary MgO-FeO-Al 2 O 3 plots of biotite show a clear calcalkaline affinity ( Fig. 4.7). In the Fe 2+ - Fe 3+ - Mg diagram (Fig: 4.8), majority of biotites plot in the ilmenite granite with few plotting in magnetite granite. Amphibole: Chemical analyses of amphiboles present mainly in diorites exposed near Dehnow is presented in Table 4.7. The SiO 2 content in amphiboles vary from to wt.% with Al 2 O 3 content varying from 8.87 to wt.%. The TiO 2 content vary from 0.74 to 1.52 wt.% with very low concentration of MnO ( < 0.73 wt.%) and Cr 2 O 3 ( < 0.04 wt.%). Chemical analyses of amphiboles when plotted on Si Vs Na+K diagram indicate that their composition vary from pargasite to tschermakite ( Fig.4. 9). In the Si Vs Na+K+Ca plots amphiboles show clear magmatic character with none of the amphiboles plotting in the field of post-magmatic amphiboles ( Fig. 4.10). Epidote: Chemical analyses of epidotes wt.% are reported from different types of granitoids along with structural formula in Table 4.8. Epidotes occur both as magmatic mineral as well as an alteration product of plagioclase in many granitoids. They show variation in SiO 2 content from to wt.% with Al 2 O 3 content varying from to wt.%. The amount of

6 TiO 2 ( < 0.66 wt.%), MnO ( < 0.43 wt.%), Na 2 O ( < 0.03 wt.%), K 2 O (0.03 wt.%) and Cr 2 O 3 ( < 0.08 wt.%) is very low ( Table 4.8). Chemistry of tourmaline, muscovite and sphene is presented in Table 4.9. Tourmaline show pleochroism from light yellowish blue to yellowish brown in colour suggesting higher amount of FeO ( wt.%). Chemical composition of magnetite and ilmenite is presented in Table P-T ESTIMATES Presence of plagioclase coexisting with amphibole and plagioclase with K-feldspar in many granitoids are well suited for the estimation of pressure and temperature during which they crystallized from the magma. Amphibole occur as an essential rock forming minerals in a wide variety of igneous and metamorphic rocks and they are especially abundant in calk-alkaline plutonic rocks. The common occurrence of amphiboles in granitoids indicate to its stability in hydrous magmatic environments. Since most of the above mineral pairs present in Mashhad granitoids exhibit magmatic texture and amphibole show typical magmatic chemistry, the composition of these mineral pairs can be used to calculate P-T conditions during which the minerals have crystallized from the magma. Apart from these, the mineral plagioclase in diorites show good magmatic zoning in support of the evidence that these are magmatic minerals crystallized directly from the melts. In the present study amphibole-plagioclase geothermobarometers and Two feldspar thermometers are being used to estimate the P-T conditions of formation of Mashhad granitoids Amphibole-Plagioclase thermobarometry Experimental studies have shown that amphiboles can be synthesized from a wide range of starting material over a pressure range of 1 to 23 kbar and at 400 to 1150 º C. This data suggest that amphibole has a considerable potential as an indicator of amphibole crystallisation condition at wide range of

7 P-T conditions. However, the compositional complexity in amphiboles has precluded its accurate thermodynamic evaluation ( Graham and Navrotsky, l984, Blundy and Holland, l990). The Al content of hornblende is not only a function of pressure but also temperature, mainly through an edenitic exchange, involving the substitution of Al for Si in the T site coupled with Na and K substitution for vacancies in the A site (Blundy and Holland, 1990). Pressures in hornblende-plagioclase can be calculated on the basis of temperature-independent barometric model given by Schmidt (1992), for the following reaction phlogopite + 2 quartz + 2 anorthite = tremolite + orthoclase + tschermakite exchange. Pressures can be estimated using the following reaction: P (±0.6kbar) = Al tot, r 2 = 0.99 Where Al tot is the Al content of hornblende in atoms per formula unit (apfu). The Al tot content of hornblende can be used to determine the depth within the pressures ranging from 2.5 to 13 kbar with a precision of ±0.6 kbar (Schmidt, 1992). Schmidt s calibration data set and the mineral assemblages he has used in his paper are consistent with the samples of granitoids of Mashhad area. This has prompted us to use the above geobarometric model to obtain information on the probable depth under which the granitoids have been formed. Since the mineral chemistry of all the amphiboles in the Mashhad granitoids are of magmatic origin which are associated with magmatic plagioclase, the mineral assemblages present in the granitoids are well suited for the application of the above barometric model. Barometric model of Schmidt is applied to obtain the pressure input required to formulate the temperature using the model of Blundy & Holland (1990) and Holland & Blundy (1994). Blundy and Holland (1990) thermometric model is applied to the amphibole-plagioclase assemblages with silica saturated rocks based on the reaction: Edenite + Quartz = Tremolite + Albite. Modification of the this simple thermometric model of Blundy and Holland(1990) to the new two thermometric models of Holland

8 and Blundy (1994) based on non-ideal mixing in amphibole and plagioclase have overcome many problems in temperature calibration and extended the formulation over a wide range of amphibole-plagioclase parageneses. The modified model comes with accuracy in calculation of error ±40 C in the range of ºC and 1 to 15 kbar, over a wide range of compositions. It is applicable to silica saturated and silica undersaturated rocks. Holland and Blundy (1994) calibrated the thermometer and defined the following conditions for using it: 1) Temperatures calculated can be in the range of ºC 2) amphiboles should have Na A >0.02 pfu, Al VI < 1.8 pfu. 3) Si in amphibole shall be in the range of pfu and 4) plagioclases with X An <0.90. Amphibole analyses from our area is extremely in accordance with the referred conditions and thus temperature is calculated on the basis of this thermometric model. Thermometric calculation is based on the reaction: Edenite + Quartz = Tremolite + Albite Site allocations used for the thermometric calculation (after Holland & Blundy,1994): A site = Na, K M4 site = Na, Ca M1,3 site = Fe 2+,Mg M2 site = Fe 2+, Mg, Al, Fe 3+ T1 site = Al, Si T2 site = Si Edenite-tremolite thermometer (for assemblages with quartz) is used in the calibration of temperature, is given by: Where the Y ab term is given by: for X ab >0.5 then Y ab = 0 Otherwise Y ab = 12.0(1-X ab ) kj

9 (Where T is the temperature in Kelvin and P is the pressure in kbar). Application of hornblende-plagioclase geobarometer and thermometer for the Mashhad granitoids give pressure estimates of 4.93 to 5.47 kbar and temperature estimates of 604 to 716 C ( Table 4.11) Two Feldspar thermometry Numerous thermometric models involve two feldspar and ternary feldspar methods which have been developed by various workers in recent years are based on the neighboring feldspar grain compositions. Elkins and Grove (1990) offer three calibrations for each feldspar pair based on exchange of albite, anorthite and orthoclase components respectively. Based on the calibration data of Elkins and Grove (1990), Putirka (2008) developed a precise and simpler thermometric model which delineates the systematic errors in the thermometric calculation. T- estimates are highly sensitive to even small changes in X Ab, X An and X Or, which are required to produce a reliable thermometer (Putirka 2008). Feldspar compositions of our samples fall well within the applicable conditions of the thermometric model which is consistently applicable to most of the igneous systems. The temperatures were retrieved by applying feldspar thermometry to plagioclase rims and coexisting K- feldspars to obtain the accurate values. Temperature is calculated based on the equation: The temperature model gives an error in calculation of ±30ºC. Applying the above equation a higher temperature estimate of 770 to 779ºC has been obtained at 5 kbar using the model of Elkins and Grove (1990) and Putirka (2008) for monzogranites. However, two-feldspar thermometer yields a lower temperature estimate of 403ºC and 379ºC for tourmaline granites and pink granites (Table 4.11).

10 Table: 4.11 Pressure-Temperature estimate for Mashhad Granitoids Rock name Minerals T P Reference Diorite Holland T and Blundy J (1994), Hbl-pl C 4.93 kb M 13/1 Schmidt(1992) Diorite Holland T and Blundy J (1994), Hbl-pl 716 C 5.47 kb M 15/1 Schmidt(1992) Monzogranite Elkins and Grove(1990), Pl-Kfld 770 C 5 kb 5/1 Putirka (2008) Monzogranite Elkins and Grove(1990), Pl-Kfld 779 C 5 kb 6/1 Putirka (2008) T-Granite Elkins and Grove(1990), Pl-Kfld 401 C 5 kb M 1/1 Putirka (2008) Pinkgranite Elkins and Grove(1990), Pl-Kfld 378 C 5 kb M 8/3 Putirka (2008)

11 Table4.1 : Electron Microprobe Analyses (EPMA) wt% of zoned plagioclase in Diorite, Mashhad, NE, Iran Dehnow Sample No : M-13/1 Core rim SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total Structural Formula on the basis of 8 Oxygen atoms Si Ti Al Fe Mn Mg Ca Na K Cr Total An Ab Or Lab Lab Byt Lab Lab Ads Ads Ads

12 Table 4.2 : Electron Microprobe Analyses (EPMA) wt % of zoned plagioclase in Granodiorite, Mashhad, NE, Iran Sample No. M- 18/1 Core rim SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total Structural Formula on the basis of 8 Oxygen atoms Si Ti Al Fe Mn Mg Ca Na K Cr Total An Ab Or Lab Lab Lab Lab Ads Ads

13 Table 4.4 : Electron Microprobe Analyses (EPMA) wt% of plagioclase in Diorite, Mashhad, NE, Iran Sample No. M 15/1 M 13/1 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Total Structural Formula on the basis of 8 Oxygen atoms Si Ti Al Fe Mn Mg Ca Na K Total An Ab Or Ads Ads Ads Lab Lab Ads Olg

14 Table 4.5: Electron Microprobe Analyses (EPMA) wt% of K-feldspar in Mashhad Granitoids, NE, Iran. Rock Name : Monzo Monzo Monz Pink Granite T-granite Sample No. M 5/1 M 6/1 M 8/1 M 8/3 M 1/1 SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Total Structural Formula on the basis of 8 Oxygen atoms Si Ti Al Fe Mn Mg Ca Na K Total An Ab Or

15 Table 4.7: Electron Microprobe Analyses (EPMA) wt% of amphibole in diorite, Mashhad, NE, Iran. M 13.1 M 13.1 M 15.1 M 15.1 SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO Na 2 O K 2 O Total Structural formula on the basis of 23 oxygen atoms Si Ti Al Cr Fe Fe Mn Mg Ca Na K Total X Mg

16 Table 4.8 : Electron Microprobe Analyses (EPMA) wt % of epidote in Mashhad Granitoids, NE, Iran Rock name : Sample No : Diorite M-5/1 Diorite M-13/1 Granodiorite M-18/1 Granodiorite M-18/1 Granodiorite M-18/1 Granodiorite M-18/1 Monzogranite M-6/1 SiO TiO A FeO MnO MgO CaO Na 2 O K 2 O Cr 2 O Total Structural Formula on the basis of 13 oxygen atoms Si A Ti Fe Mn Mg Zn Ca Na K Cr , Total

17 Table 4.9 : Electron Microprobe Analyses (EPMA) wt% of tourmaline, muscovite and sphene in Granitoid of Mashhad, NE, Iran. Rock Name Tur-granite Tur-granite monzogranite Sample No: M 13/1 M 15/1 M 6/1 Mineral Name Tourmaline Muscovite sphene SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O ZnO Cr 2 O BaO Total Structural Formula on the basis (31 O) (11 O) (20 O) Si Al o.287 Ti Fe Mn Mg Zn Ca Na K Cr Ba Total

18 Table 4.10: Electron Microprobe Analyses (EPMA) wt % of magnetite and ilmenite in Mashhad Granitoids, NE, Iran Rock Name Monzogranite Leucogranite Sample No. M-8/1 M24/1 Mineral Name Magnetite Ilmenite SiO TiO AI 2 O FeO MnO MgO CaO Cr Total Structural Formula on the (4 O) (6 O) basis of Si 0 0 A Ti Fe Mn Mg Ca cr Total

19 Fig: 4.1 Plot of anorthite content in core and rim of plagioclase in diorite Fig: 4.2 Plot of anorthite content in core and rim in plagioclase in granodiorite

20 Fig: 4.3 Plot of biotite from Mashhad granitoid on ASF diagram (after Lambert, 1959) A = (100*Al)/(Si+Al+Fe+Mn+Mg) S = (100*Si)/(Si+Al+Fe+Mn+Mg) F = [100(Fe+Mg+Mn)]/(Si+Al+Fe+Mn+Mg) Fig: 4.4 MgO(wt%) vs FeO(wt%) for biotite from Mashhad granitoid plotted in discrimination diagram (Abdel-Rahman,1994 ).

21 Fig: 4.5. Al2O3(wt%) vs MgO(wt%) for biotite from Mashhad granitoid Fig: 4.6. Al2O3(wt%) vs FeO(wt%) for biotite from Mashhad granitoid A: Anorogenic alkaline suites: Calc-alkaline orogenic suites, P: Peraluminous suites

22 Fig: 4.7. Composition of biotite from Mashhad granitoids plotted in the discrimination diagrams of Abdel Rahman et al A: Alkaline, P: Peraluminous,C: Calc-alkaline0 Fig: 4.8. Composition of biotite from Mashhad granitoids plotted in the Fe 2+ - Fe 3+ - Mg diagram of Wones & Eugster, 1965.

23 Fig: 4.9. Classification of amphibole on the basis of ratio of an +K to Si (Leake 1978). Fig: Ratio of (Ca +K +Na) to Si determining magmatic and post magmatic amphibole, (Leake 1978).