Petrology and geochemistry of the Alut granitoid complex, Sanandaj-Sirjan Zone, nw Iran

Honarmand M. and Omran N.R.

Department of Geology, Tarbiat Modares University, Tehran, Iran

mary.honarmand@gmail.com

The Zagros orogenic belt resulted form the collision between the Arabia and Eurasia plates [1,3]. It belongs to the large Alpine–Himalayan mountain chain, also referred to as the Tethysides orogenic belt [2]. This collision cause to form two presumably subduction-related arcs parallel to the Main folded-thrust Zagros which named Mesozoic Sanandaj–Sirjan and the Tertiary Urumieh–Dokhtar magmatic belts. The Alut granitoid complex is located in the northwestern part of the Sanandaj–Sirjan zone. This plutonic complex is located in a shear zone and hosted within the low to medium grade metamorphic (phyllite and schist) rocks.

This complex comprises of three main units i.e. granodiorite to monzogranite bodies (SiO₂ = 65–77 wt %) which is widespread throughout the area, a quartz-diorite to tonalite unit (SiO₂ = 52–63 wt %) exposed as two stocks and mylonitic granitoid scattered as separate outcrops through the area. NE-SW trending microquartz-diorite and NW-SE trending aplite dykes are also present in this complex.

Geochemically this complex is metaluminous to slightly peraluminous, typical of I-type granites. It belongs to low K (and high K for some monzogranite samples) calc-alkaline series and displays the geochemical characteristics typical of volcanic arc plutons related to an active continental margin area (e.g. significant Nb, Ti, P and Sr depletion). The quartz-diorite to tonalite unit has relatively high CaO, FeO, MgO, Al₂O₃ and low Rb/Sr, Sr/Y, and (La/Yb)N, which preclude an origin of variably fractionated mantle melts and favour a mafic lower crustal source. This suite is characterized by relatively low Mg# (52), various Na₂O content (2-7 wt %) and a relatively high abundance of (LILE) incompatible elements. Quartz diorite chondrite-normalized REE patterns are relatively flat and show small or no negative Eu anomalies (Eu/Eu* = 0.9 in average). However, chondrite-normalized REE patterns in granodiorites show fractionated HREE patterns and more negative Eu anomalies (Eu/Eu* = 0.5 in average) than quartz diorite pluton. Some dioritic enclaves in the granodiorites show mingling textures that may support local crustal contamination of the granodioritic melt. Dehydration melting of biotite-bearing metasedimentary sources at relatively low pressures is proposed for the origin of granodiorite to monzogranite unit.

Based on collected geochemical data, the Alut granitoids originated by partial melting of crustal protoliths having different compositions in a deformed active margin.

References:

1. Agard, P., Omrani, J., Jolivet, L., Mouthereau, F., 2005. Convergence history across Zagros (Iran): constraints from collisional and earlier deformation. International Journal of Earth Sciences 94, 401–419.

2. Sengor, A.M.C., 1987. Tectonics of the tethysides. Annual Review of Earth and Planetary Sciences 15, 213–244.

3. Sengor, A.M.C., 1992. The palaeo-Tethyan suture: a line of demarcation between two fundamentally different architectural styles in the structure of Asia. Island Arc 1, 78–91.

P-T parameters of lithosphere mantle beneath Siberian craton: evidence by kimberlite pipe Obnajennaya

Kalashnikova T.V.

A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia

Kalashnikova@igc.irk.ru

The Obnajennaya pipe is located within Kuoika field (Olenek province in northeastern Yakutian kimberlite province). The pipe was discovered in right riverbank of Kuoika River in 1957. It is unique natural cross-section of pipe uncovered from three sides by river and weathering. Pipe Obnajennaya don’t contain diamonds, but it’s interesing (attractive) by xenoliths’ abundant and variety. Initially Obnajennaya xenoliths were described by Milashev [5]. They account for 0,3-0,7 % from whole rock. It is observed the higher content of peridotites without garnets (on the one hand) and garnet pyroxenites (from the other hand) [12].

After development of geothermobarometry methods the mantle xenoliths from this pipe were determined for P-T parameters [1,7,8,10,11,12]. There are immediate and indirect methods for temperature-pressure conditions determination. Immediate methods involve decrepitation and homogenization of gas-liquid inclusions. However, the approach use restrictedly for kimberlites through occuring hydrothermal-metasomatic processes and low-temperature alteration. Indirect methods enclose calculations, applying dependence syngenetic mineral analyses from P-T conditions.

It may be distinguished 4 groups of geothermobarometers by mineralogical basis (accordingly [4]). It is recognised olivinic, pyroxenic, pyroxene-garnet and pyroxene-spinel thermobarometers. Olivinitic thermometers use Mg²⁺-Fe²⁺ distribution between olivine and another syngenetic minerals (e.g Ol-Grt, Ol-Sp in O’Neil, Wood, 1979). For barometry Al distribution exploits (e.g. Ol-Grt in Brey, Kohler, 1990). Pyroxenic thermobarometers are the most various, using different elements partitioning in pyroxenes equivalent structural position (e. g. Ca-Mg in Opx-Cpx solvus [3]; Ca in Opx – Mercier, Wells, 1985; Cr in Cpx with coexisting garnet – [6]). Pyroxene – garnet thermometers apply Fe-Mg distribution between some pyroxene and garnet (e. g. Ellis, Green, 1979; Harley, 1984, Krogh, 1988 - according [4]).

Previous researchers of pipe Obnajennaya have used different geothermobarometers. Ukhanov with co-authors applied geothermometer of Ellis, Green, 1979 and Ryabchikov, 1975. The using of Opx-thermometer produces lower results so temperature of overlapping enstatisitation and exsilution of pyroxene is knowingly lower than garnet crystallization temperature. The authors received following sequence: peridotites without garnet (15-16 kbars; 50 km of depth) – high-magnesium garnet pyroxenites (13-23 kbars; 40-75 km) – eclogites (23-30 kbars; 75-100 km) – Fe-Mg garnet peridotites. Solov’eva with co-authors practiced geothermometer of Finnerty, Boyd, 1984 (Ca-Mg in two-pyroxene solvus) and barometer of MacGregor, 1974 (Al distribution in Opx). This barometer can’t use in spinel field. The following series was obtained: coarse-grained lherzolites – websterites – deformed lherzolites. Taylor with co-authors used pyroxene – garnet thermometers for eclogites and get the next P-T parameters – 711-923°C and 21-37.6 kbar. The most up-dated work was carried out by T. Alifiriva with co-authors, presented on 10 IKC. Theirs data for pyroxenites were 690-910°C and 20-45 kbars.

Therefore, different thermobarometers indicate equilibrium conditions for various reactions. We used a few thermobarometers for Obnajennaya mantle xenolith.

Our collection includes 35 samples: 16 peridotites (4 – spinel harzburgites; 7 – coarse-grained garnet-spinel lherzolites; 5 – websterites), 8 – coarse-grained garnet pyroxenites, 5 – eclogites, 6 – mica-bearing metasomatites. The most of them have porphyroblastic structure. Serpentinised pyroxenes and olivines were signed mainly in lherzolite-harzburgites series. In individual grains pyroxenes represented exsolution structure with clynopyroxene and spinel. In 3 samples of pyroxenites thin garnet segregation between pyroxenes are observed. P-T conditions were estimated by means of BKN [3] and NT [6] thermobarometers. The data of NT-NT is considerably lower. Fe-Mg thermobarometers give approximate results. Grt-pyroxenites obtained 500-600°C and 7-15 kbars, the results are the lowest. The values of Sp lherzolites are some higher (630-820°C and 18-20 kbar), the data for Sp-Grt lherzolites are similar (600-900°C; 21-26 kbar). But one Ilm-Phl metasomatite gives the highest results (1143°C; 40,9 kbar). It may be explained by mantle metasomatism. The relict grains of pyroxene have the deepest origin from the sampling.

By the results the geothermal of Obnajennaya pipe was constructed. In general our data correspond with earlier evidence. The data is more lower than evidence for Udachnaya xenoliths [2,7,9]. In Obnajennaya the content of deformed xenoliths is lower, lherzolite and Sp-bearing variety is dominated Lherzolites and harzburgites way be interpterted as the results of restite after melting of basalt and komatiite magmas.. The Garnet variety has subordinate content. It may be point out the different ages of these pipes (Obnajennaya – 180 MA – upper Mesozoic and Udachnaya –Paleozoic) and distinctions reflect lithosphere mantle conditions at different times.

References:

1. Alifirova TA, Pokhilenko LN, Malkovets VG and Griffin Wl (2012) Petrological inferences for the role of exsolution in upper mantle: evidence from the Yakutian kimberlite xenoliths - Proceeding of 10IKC - №048 (CD-R disk).

2. Boyd, F.R., Pokhilenko, N.P., Pearson, D.G., Mertzman, S.A., Sobolev, N.V., Finger, L.W. (1997). Composition of the Siberian cratonic mantle: evidence from Udachnaya peridotite xenoliths. Contributions to Mineralogy and Petrology 128, 228-246.

3. Brey, G.P., Kohler, T. (1990) Geothermobarometry in fourphase lherzolites II: New thermobarometers and practical assessment of existing thermobarometers. - J. Petrol., v. 31, pp. 1353-1378.

4. Menaker I.G. (1993) Geothermobarometry of ultrabasic rocks (in Russian)– Nauka, Novosibirsk.

5. Milashev VA (1960) The related inclusions from kimberlite pipe Obnajennaya – Proceeding of Sovetian Mineralogical Society (in Russian) - V.89, №3 – pp. 284-299.

6. Nimis, P., Taylor, W.R. (2000). Single Clinopyroxene thermobarometery for garnet peridotites. Part 1, Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-cpx thermometer. - Contributions to Mineralogy and Petrology, № 139 - pp 541–554.

7. Pokhilenko, N.P., Sobolev, N.V., Kuligin, S.S., Shimizu, N. (1999). Peculiarities of distribution of pyroxenite paragenesis garnets in Yakutian kimberlites and some aspects of the evolution of the Siberian craton lithospheric mantle, in: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.), Proc. 7th Internatl. Kimberlite Conf. RedRoof Design, Cape Town, pp. 689-698.

8. Qu Qi, Lawrence A. Taylor, Gregory A. Snyder & Nikolay V. Sobolev (1994) Eclogites from the Obnazhennaya Kimberlite Pipe, Yakutia, Russia. - International Geology Review, 36:10, 911-924

9. Sobolev, V.S., Sobolev, N.V. (1964) Xenoliths in kimberlites of Northern Yakutia and upper mantle structure (in Russian). Doklady AN SSSR, v. 158, pp. 22-26.

10. Solov’eva L.V., Vladimirov B.M. et al .(1994) Kimberlite and kimberlite – similar rocks: matter of upper mantle beneath ancient platforms (In Russian) - Nauka, Novosibirsk.

11. Taylor, L.A., Snyder, G.A., Keller, R., Remley, D.A., Anand, M., Wiesli, R., Valley, J., Sobolev, N.V. (2003) Petrogenesis of group A eclogites and websterites: Evidence from the Obnazhennaya kimberlite, Yakutia. - Contrib. Mineral. Petrol., v. 145, pp. 424-443.

12. Ukhanov AV, Ryabchikov ID, Khar’kiv AD (1988) Lithospheric mantle of Yakutian kimberlite province (in Russian). - Nauka, Moscow.