Petrogenetic modeling of rock variety in the Khalkhab–Neshveh pluton,NW of Saveh,Iran

The Khalkhab–Neshveh (KN) pluton is a part of Urumieh–Dokhtar Magmatic Arc and was intruded into a covering of basalt and andesite of Eocene to early Miocene age. It is a medium to high‐K, metaluminous and I‐type pluton ranging in composition from quartz monzogabbro, through quartz monzodiorite, granodiorite, and granite. The KN rocks show subtle differentiation trends strongly controlled by clinopyroxene, plagioclase, hornblende, apatite, and titanite, where most major elements (except K₂O) are negatively correlated with SiO₂; and Al₂O₃, Na₂O, Sr, Eu, and Y define curvilinear trends. Considering three processes of magmatic differentiation including mixing and/or mingling between basaltic and dacitic magmas, gravitational fractional crystallization and in situ crystallization revealed that the latter is the most likely process for the evolution of KN magma. This is supported by the occurrence of all rock types at the same level, the lack of mafic enclaves in the granitoid rocks, the curvilinear trends of Na₂O, Sr, and Eu, and the constant ratios of (⁸⁷Sr/⁸⁶Sr)_i from quartz monzodiorite to granite (0.70475 and 0.70471, respectively). In situ crystallization took place via accumulation of plagioclase and clinopyroxene phenocrysts and concentration of these phases in the quartz monzogabbro and quartz monzodiorite at the margins of the intrusion at T ≥ 1050°C, and by filter pressing and fractionation of hornblende, plagioclase, and later biotite in the granitoids at T = ～880°C.     

Metasomatism of gabbro – mineral replacement and element mobilization during the Sveconorwegian metamorphic event

Widespread metasomatism affected the 100 km long and 25 km wide Proterozoic Bamble and Modum‐Kongsberg sectors, South Norway, resulting in the chemical and mineralogical transformation of wide segments of continental crust. Scapolitization was associated with veining, and was followed by albitization, transforming metagabbros pervasively over large areas. Fluids played an active role in these reactions, forming H₂O‐, CO₂‐ and Cl‐bearing phases at the expense of the primary volatile‐free minerals, causing depletion in Fe and infiltration of K, Mg, Na, B and P. The transformation of gabbro to scapolite metagabbro is observed as a fluid front replacing the primary magmatic mineral assemblage in three stages: during an incipient amphibolitization stage, the primary mafic minerals were replaced by anthophyllite or hastingsite, followed by pargasitic and edenitic Ca‐amphibole. Magnetite was dissolved, while rutile formed by the breakdown of ilmenite. Plagioclase was replaced by Cl‐rich scapolite (Me_19‐42) reflecting Cl‐saturation, while K‐ and Mg‐saturation produced phlogopite, enstatite, sapphirine and rare corundum. The high modal contents of chlorapatite and tourmaline in the scapolite metagabbro imply infiltration of B and P. The albitites consist dominantly of albite (Ab_95‐98) with varying, generally small, amounts of chlorite, calcite, rutile, epidote and pumpellyite. They formed from a H₂O–CO₂‐fluid rich in Na. The gabbro yields a zircon U–Pb age of 1149 ± 7 Ma and tonalite 1294 ± 38 Ma, whereas rutile from scapolite metagabbro and albitite has U–Pb ages of 1090–1084 Ma, and phlogopite produced during scapolitization Rb–Sr ages of 1070–1040 Ma. Temperature conditions for the scapolitization are inferred to have been 600–700 °C. The reported ages, combined with mineralogical and petrographic observations and inferred P–T conditions, indicate that the metasomatism was a part of the regional Sveconorwegian amphibolite facies metamorphic phase. Initial ⁸⁷Sr/⁸⁶Sr of the scapolite ranges from 0.704 to 0.709. The Sr‐signature, the Cl‐ and B‐rich environment and regional distribution of lithologies suggest that the fluid may have originated from evaporites that were mobilized during the regional metamorphism.     

THREE NATURAL ZIRCON STANDARDS FOR U-TH-PB, LU-HF, TRACE ELEMENT AND REE ANALYSES

We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.     

U-Pb Age, Setting and Tectonic Significance of the Anorthosite-Mangerite-Charnockite-Granite Suite, Lofoten-Vesteralen, Norway

Mangerites, charnockites, anorthosites, gabbros and granitesoccur within a high-grade metamorphic complex in the Lofoten–Vesterålenislands of northern Norway. U–Pb dating of zircon, titaniteand monazite indicates a three-stage magmatic history beginningat 1870–1860 Ma with the emplacement of the Lødingenand Hopen plutons, followed by a dominant stage at 1800–1790Ma that formed the bulk of the suite, and concluded by the emplacementof pegmatites, local rehydration and retrogression between 1790and 1770 Ma. On the scale of the Baltic Shield the 1870–1860Ma episode corresponds to contraction, amalgamation of arcs,and regional deformation. By contrast, the episode at 1800–1790Ma was characterized by major shifts in plate convergence, byintraplate deformation, and by a diversity of magmatic associationsincluding suites derived from the subcontinental mantle andwidespread granitoid rocks extracted from the continental crust.The diversity of concurrent magmatic events across the Svecofennianorogen, and the temporal coincidence with collisional eventsin coeval orogenic belts, suggests that the genesis of the suiteof magmatic rocks may have been related to tectonically drivenmechanisms of magma generation. KEY WORDS: anorthosite–mangerite–charnockite–granite; lithospheric processes; Lofoten–Vesterålen; Svecofennian orogen; U–Pb geochronology     

Age and Petrogenesis of Two Late Archean Magmatic Suites, Northwestern Superior Province, Canada: Zircon U-Pb and Lu-Hf Isotopic Relations

The paper presents U–Pb ages for zircon, titanite, andmonazite, and Hf isotopic data for zircon, from the rocks oftwo magmatic suites occurring mostly in the Archean Uchi Subprovinceand partly in the neighbouring Berens River and English Riversubprovinces of the northwestern Superior Province, Ontario.These data, together with observations on the morphologies and,where evident, the inheritance of the zircon crystals, constrainthe nature of the sources of the magmas and provide a recordof various crustal processes in their evolution. The older of the two magmatic suites formed at 2744–2740Ma along segments of a common arc system. The suite consistsof (1) several trondhjemitic to granodioritic plutons, with_Hf values of 6•1, intruded into older crust and possiblyformed from magma produced by partial melting of subducted,juvenile oceanic crust; (2) an assemblage of dacitic pyroclasticvolcanic rocks, with _Hf values of 3•2–4•0, associatedwith tholeiitic basalts and probably derived from magma meltedfrom arc mantle; and (3) a bimodal assemblage of tholeiiticbasalts, rhyolites, and porphyries, also with _Hf values of 6•1,associated with a volcanogenic massive sulphide deposit andapparently formed by differentiation of mantle-derived basalticmelts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma,comprises late orogenic plutons and batholiths of dioritic todominantly granodioritic composition. The characteristics ofthese intrusions are consistent with a process combining meltingof a metasomatized mantle source and subsequent fractional crystallizationof the derived magmas at shallow depths. However, most of thestudied occurrences show evidence of crustal contamination throughvarious combinations of assimilation of lower-crustal material,assimilation of underthrust sedimentary rocks, and contaminationby wall rock materials during the latest stages in the emplacementof the plutons. The involvement of crustal material is indicatedby the presence of zircon xenocrysts and by _Hf values rangingfrom 1•4 to 4•4. Only one intrusion, with an _Hf valueof 5•0 and no xenocrystic zircon, appears to have escapedwidespread contamination, perhaps because the ascent of itsmagma was facilitated by a crustal-scale fracture system.