Petrogenesis of the Lalezar granitoid intrusions (Kerman Province – Iran)

The Lalezar granitoids crop out within volcanic successions of the Urumieh-Dokhtar Magmatic Assemblage (UDMA). These granitoids have a range from gabbro-diorites to granites in composition. The mineral compositions of the most felsic rocks are characterized by the abundances of Na-plagioclase, quartz, alkali feldspar, biotite and hornblende. In the gabbro-diorite rocks, plagioclase (Ca-rich), hornblende, biotite and clinopyroxene are the most common minerals. Major element geochemical data show that the Lalezar granitoids are mostly metaluminous, although the most felsic members (granites) attain slightly peraluminous compositions and that they have features typical of high-K calc-alkaline rocks. In primitive mantle-normalized trace element spider diagrams, the analysed samples display strong enrichment in LILE compared to HFSE, accompanied by negative anomalies of Nb,Ta and Ti. REE chondrite-normalized plots show moderate LREE enrichment with slight to strong negative Eu anomalies. Rb–Sr geochronological data, mainly dependent on the Sr isotopic composition of biotite, was obtained in two samples and it points to 15-16 Ma. As a probable, age for the emplacement of the studied intrusives. Initial 87Sr/86Sr ratios and ƐNdi values range from 0.70495 to 0.70565 from +3.1 to +1.5 respectively, which fit into a supra-subduction mantle wedge source for the parental melts and indicates that, in general, crustal contribution for magma diversification was not relevant. Geochemical and isotopic evidence reveal that the Lalezar intrusions are cogenetic I-type granitoids which were generated in a continental arc setting, in agreement with models previously presented in the UDMA

Geochronology, isotope geochemistry and tectonomagmatic setting of the Lalezar granitoids (Urumieh-Dokhtar Volcanic Belt, Iran)

The Lalezar granitoids crop out within the Urumieh-Dokhtar Volcanic Belt, which is the largest volcanic belt in Central Iran. These granitoids have intruded into the Eocene volcano-sedimentary rocks and range from gabbro-diorites to granites in composition, with dominance of diorites and tonalites. Two of the least altered samples, 5-ln-7 and 23-ln-6, were selected for Rb–Sr geochronology. Biotite (Bt), hornblende (Hbl) and plagioclase (Pl) separates were obtained from both samples. For sample 5-ln-7, using the data from the whole-rock and the three mineral separates, a 87Sr/86Sr vs. 87Rb/86Sr correlation corresponding to a 14.6±5.8 Ma age is obtained, with initial 87Sr/86Sr=0.7055. However, the MSWD has a very large value (376). This is due to the fact that Hbl composition plots deviated from the alignement defined by WR, Pl and Bt, suggesting that some disturbance took place. Under the petrographic microscope, the amphibole grains in this sample show some low temperature alteration, as testified by chloritization and oxidation, which makes plausible that a late enrichment in radiogenic Sr could have affected hornblende. If Hbl is discarded, the result now is a 15.0±0.4 Ma Bt-Pl-WR isochron, with MSWD=2.4 and initial 87Sr/86Sr=0.70517. Considering the errors, both results (with and without Hbl) overlap, which suggests that there was Sr isotope equilibrium at an age of ca. 15 Ma (most likely during igneous crystallization). In the 87Sr/86Sr vs. 87Rb/86Sr diagram for sample 23-ln-6, the line obtained with Bt-Hbl-Pl-WR has a slope indicating an age of 15.8±1.6 Ma. The MSWD value is 18 and the initial 87Sr/86Sr ratio is 0.70533. The MSWD shows that the correlation is not perfect, and, as in the previous case, it probably reflects some minor alteration; once again, Hbl plots above the line that passes through WR, Pl and Bt. Taking the errors into account, the ages calculated for 23-ln-6 and 5-ln-7 overlap each other, suggesting that this set of data is geochronologically meaningful. Therefore, and considering that the studied rocks are shallow intrusives which should have not undergone a long cooling period, the obtained 15-16 Ma ages are probably dating the intrusive events. For isotope geochemistry, Sr and Nd isotopic compositions were determined for 14 whole-rock samples. Assuming an age of 15 Ma, initial 87Sr/86Sr and εNd values vary in restricted ranges from 0.70495 to 0.70565 and from +3.1 to +1.5, respectively. In the εNdi versus (87Sr/86Sr)i diagram, this cluster plots to the right of the so-called mantle array and overlaps the field of island-arc basalts. The limited range of Sr and Nd isotopic compositions suggest that the Lalezar intrusions are co-genetic, deriving from the same parental magmas essentially by magmatic differentiation processes. Taking into account the IAB-like isotopic compositions of the studied rocks, the parental magmas may have been formed by partial melting in a supra-subduction mantle wedge. The occurrence of gabbrodioritic rocks in the Lalezar suite provides additional evidence in favour of an origin of the parental magmas by melting of mantle peridotites, rather than by melting of mafic crust

Effect of Varying Normal Stiffness on Soft Rock Joints under Cyclic Shear Loads

The evaluation of changes in shear resistance on soft (or weathered) rock joints under cyclic shear loads with constant normal load (CNL) and constant normal stiffness (CNS) significantly contributes to increasing the safety and stability of rock slopes and underground structures. In this study, a series of cyclic shear tests were conducted on simulated soft rock joints with regular (15°-15°, 30°-30°) and irregular (15°-30°) asperities under different normal stiffnesses (kn). The results indicated that the first peak shear stress increases with the increase in kn up to the normal stiffness of the joints (knj). Beyond knj, no significant change was observed in the peak shear stress. The difference in peak shear stress between regular (30°-30°) and irregular joints (15°-30°) increases as kn increases. The minimum difference of peak shear stress between regular and irregular joints was observed (8.2%) under CNL and the maximum difference was found (64.3%) on knj under CNS. The difference in peak shear stress between the first and subsequent cycles significantly increases as both the joint roughness and kn increases. A new shear strength model is developed to predict peak shear stress of the joints for different kn and asperity angles under cyclic shear loads