The Middle-Late Cretaceous Zagros ophiolites, Iran: Linking of a 3000 km swath of subduction initiation fore-arc lithosphere from Troodos, Cyprus to Oman

New trace-element, radiogenic Sr-Nd-Pb isotopic and geochronological data from Middle-Late Cretaceous Zagros ophiolites of Iran give new insights into the tectono-magmatic history of these supra-subduction zone (SSZ)-type ophiolites. The distribution of Middle-Late Cretaceous SSZ-type ophiolites in Iran comprises two parallel belts: (1) the outer Zagros ophiolitic belt and (2) the inner Zagros ophiolitic belt. These Middle-Late Cretaceous ophiolites were generated by seafloor spreading in what became the fore-arc and back-arc during the subduction initiation event and now define a ∼3000-km-long belt from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. The Zagros ophiolites contain complete (if disrupted) mantle and crustal sequences. Mantle sequences from both outer-belt and inner-belt ophiolites are dominated by dunites, harzburgites, and lherzolites with minor chromitite lenses. Peridotites are also intruded by gabbros and a variety of mafic to minor felsic (plagiogranite and dacite) dikes. Crustal rocks comprise ultramafic-mafic cumulates as well as isotropic gabbros, sheeted dike complexes, pillowed and massive lavas, and felsic rocks. Our new zircon U-Pb ages indicate that the outer-belt and inner-belt ophiolites formed near coevally during the Middle-Late Cretaceous; 100−96 Ma for the outer belt and 105−94 Ma for the inner belt. Both incompatible-element ratios and isotopic data confirm that depleted mantle and variable contributions of subduction components were involved in the genesis of outer-belt and inner-belt rocks. Our data for the outer belt and inner belt along with those from better-studied ophiolites in Cyprus, Turkey, the UAE, and Oman lead to the conclusion that a broad, ∼3000-km-long swath of fore-arc lithosphere was created during Middle-Late Cretaceous time.

The Izabal Geosol- A case of arrested development of a nickel laterite in Central America

The Izabal Geosol is a new stratigraphic unit defined by the author within the Sierra de Santa Cruz (SSC) ophiolitic belt which is located to the north of Lake Izabal, in the east-central portion of Guatemala, some 140 km northeast from Guatemala City. The Geosol Izabal is composed of the following five pedosols: a. Gossan or Iron Cap (a.k.a. Ferricrete) b. Limonitic Horizon c. Stoneline Horizon d. Mottled Zone Horizon (a.k.a. Transition Zone) e. Saprolitic Horizon The Saprolitic Horizon lies over a Saprock Horizon, which, in turn, lies directly over a less weathered bedrock of the SSC ophiolitic belt. The most complete profiles are usually presented over weathered dunites and serpentinites, while on top of the less altered pyroxenites there usually forms only a limonitic horizon. The SSC ophiolitic belt overthrusted the Maya Block of the North American Continental Plate during the Early Tertiary period along a flat structure that outcrops at its base. The SSC is one of the five ophiolitic belts identified to date in Central Guatemala that are related to the northwestern border of the Caribbean Plate and, thus, to the ophiolitic belts from Cuba and Dominican Republic. The intrusives of this belt are composed of alternating layers of dunite and pyroxenites, usually strongly serpentinized. Existing data indicate that these rocks underwent fractional crystallization and probably some degree of magma mixing and contamination prior to their main tectonic event. The Izabal Geosol represents a case of arrested development of a wet lateritic profile, which develops mainly as large pockets controlled by tectonics and the presence of dunitic rocks. Its most striking difference from other wet laterites is the presence of up to 30% magnetite within the Saprolitic Horizon, which indicates relatively young age of these laterites. This magnetite carries nickel grades of 1.5 to 2% associated to nickel sulphides. It could represent a secondary product for a plant producing ferro-nickel or steel products. The presence of these large amounts of magnetite also allows the use of aeromagnetic surveys for the identification of areas of interest.

ON THE RELATIVELY COMPOSITIONAL HOMOGENEITY OF ALBANIAN EASTERN OPHIOLITIC BELT

Ophiolitic formation of Albanides, named as Mirdita zone, represents a compactsegment of oceanic lithosphere of Middle-Upper Jurassic. Based on petrographic,geochemical and metalogenical features two types of belts are distinguished: westernMORB and eastern SSZ types. In fact, structural and geological units as well as manyother elements have shed light on lack of a sharp separation between the two belts.Recent investigations have evidenced that different ultramafic massifs of westernophiolitic formation, represent an evident variation of their composition fromharzburgite to lherzolitic -types. This composition reflects a different grade of partialmelting of upper mantle. Peridotites show a high variability, from 0.3 - 3.8 wt.%Al2O3, varying from small to highly extreme depleted peridotite.On the contrary, Albanian eastern belt, it seems to be formed by a more homogeneoushartzburgitic mantle.Detailed petrologic and metallogenic investigations have evidenced that this beltchanges also from one massif to another, naturally at a smaller level, therefore it iseasier to be named relatively homogeneous. It is distinguished by a higher meltingdegree, chiefly of hartzburgitic-type, characterized by whole and thick ultramaficsection, as well as by metalogenic variety, mostly of metallurgic-type of chromitemineralization. It is supposed that rock-forming and mineraluzation processes havebeen developed not uniformly along the ophiolitic belt.