Adakitic signature of quartz monzonitic porphyry stock and related cross-cutting dikes at Kighal, nw Iran

Simmonds V.

Research Institute for Fundamental Sciences, Tabriz University, Tabriz, Iran

Simmonds_vartan@yahoo.co.uk

Quartz monzonitic porphyry stock at Kighal is located in ~12 km north of Varzeghan, East Azarbaidjan Province, NW Iran. Based on different structural-geologic classifications of Iran, this area is part of volcano-plutonic zone of Alborz-Azarbaidjan. This stock lies within the north-western part of the Cenozoic Sahand-Bazman volcano-plutonic belt of Iran, formed by subduction of the Neotethian oceanic crust beneath central Iran during the Alpine orogeny. This porphyry stock intruded upper Eocene andesitic-latitic lava flows during upper Oligocene-lower Miocene and produced hydrothermal alteration zones and Cu-Mo mineralization in the area.

Subsequently, the peripheral volcanic rocks and some parts of the Kighal stock were intruded by three barren sub-volcanic bodies during Miocene period, ranging in composition from granodiorite, through micro-diorite to monzonite. Additionally, numerous cross-cutting dikes with various compositions ranging from diorite-quartz diorite to granodiorite and microdiorite, branching from these barren bodies, intruded the porphyry stock. They have different structural trends, but the dominant trends are NW-SE and NE-SW. The thickness of these dikes varies from 0.5 to 3 m.

The porphyry stock and the cross-cutting dikes display I-type magnesian, calc alkaline to high-K calc alkaline signature and metaluminous to peraluminous geochemical character. They have emplaced within an active continental margin tectonic setting, associated with a post-collisional phase.

All the 16 samples taken from the porphyry stock and cross-cutting dikes display the typical geochemical characteristics of adakites, that is, SiO₂>56 wt% (ave. 62.26%), Na₂O content between 3.5 and 7.5 wt% (ave. 4.11%), high Sr content (>400 ppm; ave. 563.53 ppm), high Sr/Y ratio (ave. 41.14), high Mg# (~0.51; ave. 0.56), moderately high content of Fe₂O₃+MgO+MnO+TiO₂ (~7 wt.%; ave. 9.19%), high Ni and Cr contents (ave. 47.87 and 180 ppm), high LILE content (e.g., K, Rb, Ba, Th), depletion of Y (18 ppm; ave. 15.45 ppm) and Yb (1.8 ppm; ave. 1.45 ppm), HFSE (e.g., Nb and Ta) and Ti; Moreover, REE patterns exhibit fractioned pattern ((La/Yb)_N>10; ave. 18.34) with low HREE concentrations [3,4,7]. On the other hand, in Yb/SiO₂ and Y/SiO₂ diagrams, data points are plotted within adakitic rocks area. Adakitic tendency of studied samples is more obvious in La/Yb-Yb and Sr/Y-Y plots.

Based on compositional classification provided by Martin and Moyen [6] for modern adakites, the studied rocks fall into high-SiO₂ (HSA) field, as their SiO₂ content is higher than 60 wt%, MgO content is between 0.5 to 4 wt%, CaO+Na2O<11 wt.% (ave. 6.46%), Sr<1100 ppm and they are relatively Rb-rich, compared to low-SiO₂ (LSA) adakites.

HS-adakites are directly linked to slab-melts, considered as the direct result of low to moderate-degree partial melting of the subducted hydrated basalt at pressures high enough to stabilize garnet±amphibole (i.e., garnet amphibolites or eclogite), and have variably been contaminated by mantle wedge (peridotite) assimilation, as they ascend through the mantle wedge [5]. The studied adakitic rocks also display this feature, as the data points in different discriminating diagrams, such as MgO-SiO₂, Fe₂O₃-K₂O-MgO and Th/Ce-Th are plotted within the slab-derived adakites field.

The general trend in La/Yb-Yb diagram shows that partial melting, rather than fractional crystallization, was the dominant process of magma generation. Ce/Sm-Sm/Yb and Rb/Sr-Nb/Th diagrams indicate the presence of garnet and amphibole within the source materials. Meanwhile, the fractioned pattern of REE’s with low HREE content testifies to presence of garnet as residual phase within the source materials during partial melting. The negative Ti-Nb-Ta anomalies are also produced by presence of residual hornblende and/or Fe-Ti oxides [5].

Experimental studies have revealed that partial melting of metabasaltic igneous rocks of subducting oceanic slab in garnet-amphibolite to eclogite facies, can produce melts with typical characteristics of adakites (e.g. [1,3] ).

For the studied area, in Nb/Ta versus Zr/Sm plot, most of the data points fall in the field of amphibole and hornblende eclogite melting, suggesting that the crustal thickness must be high (near to 50 km). Similarly, the negative Nb, Ta and Ti anomalies of the studied rocks suggest that their source materials may have melted under high pressure in which rutile was a residual phase at a pressure of more than 1.5 GPa [2,8]. The average thickness of the NW Iran continental crust has been estimated about 40-45 km. The other evidence is that, the general pattern of trace and rare earth elements in ORG-normalized spider diagram [Pearce et al., 1984] displays a good correlation with the pattern of granitoids related to active continental margin's volcanic arcs, such as Chilean granitoids in Andes, suggesting the emplacement of intrusive bodies within highly to moderately thick continental crust.

On this basis, it can be concluded that adakitic magma in this area was produced by melting of Neotethian oceanic slab in the amphibole-eclogite or garnet-amphibolite facies under high pressures and thick continental crust.

References:

1. Atherton, M. P. and Petford, N. (1993) Generation of sodium-rich magmas from newly underplated basaltic crust, Nature, 362, 144–146.

2. Barth, M. G., Foley, S. F. and Horn, I. (2002) Partial melting in Archean subduction zones: constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonalitic melts under upper mantle conditions, Precambrian Research, 113, 323–340.

3. Defant, M. J. and Drummond, M. S. (1990) Derivation of some modern arc magmas by melting of young subducted lithosphere, Nature, 347, 662-665.

4. Martin, H. (1999) The adakitic magmas: modern analogues of Archaean granitoids, Lithos, 46 (3), 411– 429.

5. Martin, H., Smithies, R. H., Rapp, R., Moyen, J.-F. and Champion, D. (2005) An overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution, Lithos, 79, 1-24.

6. Martin, H. and Moyen, J.-F. (2002) Secular changes in TTG composition as markers of the progressive cooling of the Earth, Geology, 30 (4), 319– 322.

7. Maury, R. C., Sajona, F. G., Pubellier, M., Bellon, H. and Defant, M.J. (1996) Fusion de la crouˆ te oce´anique dans les zones de subduction/collision re´centes: l’exemple de Mindanao (Philippines), Bull. Soc. Geol. Fr., 167 (5), 579– 595.

8. Xiong, X. L., Xia, B., Xu, J. F., Niu, H. C. and Xiao, W. S. (2006) Na depletion in modern adakites via melt/rock reaction within the sub-arc mantle, Chemical Geology, 229, 273–292.