Zircons In Rocks Research Articles

Geochronological cross section through northern Thailand

Isotopic age determinations were carried out on seven different localities along a cross section through northern Thailand. First results of UPb investigations on zircons of rocks from the crystalline basement west of Tak indicates a maximum age of some 200 Ma for the last amphibolite facies overprint instead of Precambrian. Tertiary KAr ages of around 30 Ma on mica separates of these crystalline rocks must be interpreted as cooling ages requiring tectonic movements with strong uplift in Tertiary time. Additional information of the tectono-metamorphic evolution of northern Thailand is given by KAr ages on fine mineral fractions of slates which date the peak of a regional metamorphism and the time of deformation. The low grade metamorphic overprinting in the eastern parts of northern Thailand covers a range from 250 Ma to 60 Ma indicating a long lasting deformational history. The obtained data are consistent with the idea of a continuous transpressive regime since Permian/Triassic time resulting in thin-skinned tectonics in the eastern part and ramping, shearing and strong uplifting especially in Tertiary time in the western parts.

塙深成岩体の分化とジルコンの形態との関係

The Hanawa pluton reported by Tanaka and Ochiai (1988) is devided into main bodies and small fine-grained quartz diorite-tonalite bodies. K-Ar ages using hornblende by Shibata and Utsumi (1983) are 117 Ma for fine-grained quartz diorite-tonalite bodies and 99 Ma for main bodies which intrude the former. The main bodies are devided into granodiorite, foliated granodiorite-tonalite and medium to fine-grained tonalite-granodiorite facies which are probably formed by succesive intrusion of magma differentiated at deeper level. Furthermore the granodiorite is subdivided into mylonitelike, large sphene bearing, porphyritic and fine-grained sub-facies, which are seem to be differentiated at shallower level. Each facies contains zircons showing characteristic morphology; granodiorite facies (110 dominant type), foliated granodiorite-tonalite facies (100 dominant type), medium to fine-grained tonalite-granodiorite facies (110/100 intermediate type), fine-grained quartz diorite-tonalite facies (E/F dispersed type). The morphology of zircons in each rock facies is different in response to petrochemical characters. Prism index of zircon from main bodies decreases systematically with increasing SiO2 contents of the rocks and with decreasing FeO, CaO, and MgO contents. Therefore the prism index of zircon is related to differentiation of granite magma. While prism index of zircon in fine-grained quartz diorite-tonalite of older stage is completely different from the above trend of the main bodies. On the other hand U, Th and HREE contents in the zircons of the Hanawa pluton show a wide range from several ten to several hundreds ppm. Elongation index of zircon is clearly related to concentration of these elements substituting Zr. In conclusion, morphological variations of zircons in granitic rocks reflect evolution of magma. Therefore the study of morphology of zircon is useful for understanding of petrogenesis of granite magma.

U–Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska

Metamorphic rocks within and west of the northern Coast Mountains in southeastern Alaska consist of an Upper Proterozoic(?) to upper Paleozoic continental margin assemblage that we interpret to belong to the Yukon-Tanana terrane. U–Pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of ~495 Ma, ~750 Ma, 1.05–1.40 Ga, 1.75–2.00 Ga, ~2.3 Ga, 2.5–2.7 Ga, and ~3.0 Ga. Multigrain fractions from two samples yield upper intercepts between 2.0 and 2.3 Ga, but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of < 2.0 and > 2.3 Ga grains. Zircons in these rocks generally overlap in age with (i) detrital zircons in metasedimentary rocks of the Yukon–Tanana terrane in eastern Alaska and Yukon, (ii) detrital zircons in strata of the Cordilleran miogeocline, and (iii) plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata. In addition, the pre-1.7 Ga grains overlap in age with dated crystalline rocks of the western Canadian Shield. These similarities raise the possibility that metaclastic rocks in the northern Coast Mountains accumulated in proximity to western North America. The younger zircon populations were likely shed from mid-Proterozoic to early Paleozoic igneous rocks that now occur locally (but may have been widespread) along the Cordilleran margin. Recognition of a continental margin assemblage of possible North American affinity in the Coast Mountains raises the possibility that some arc-type and oceanic terranes inboard of the Coast Mountains may be large klippen that have been thrust over the North American margin.

2.2 Ga age of zircons in three occurrences of Upper Proterozoic clastic rocks of the northern Cassiar terrane, Yukon and British Columbia

Analyses of detrital zircons from three occurrences of Upper Proterozoic clastic rocks of the northern Cassiar terrane in Yukon and northern British Columbia yield a U–Pb age of 2224 ± 22 Ma. The zircons apparently belong to a single population similar in age to zircons in stratigraphically equivalent rocks of the southern Cassiar terrane and to zircons in rocks in the Yukon–Tanana terrane of Alaska. A source terrane or area of the required age and extent has not yet been identified.

U–Pb zircon and sphene geochronology of a composite Archean granitoid batholith, Favourable Lake area, northwestern Ontario

U–Pb zircon and sphene data for several phases of composite batholiths in the Favourable Lake area of the Superior Province indicate two major periods of plutonism separated by a hiatus of over 200 Ma.A trondhjemite, which now forms a metamorphosed remnant within a batholith, was emplaced 2950 ± 5 Ma ago during an early plutonic event. The second and dominant plutonic period occurred between [Formula: see text] and 2711.0 ± 2.0 Ma ago; these are the ages of the earliest and the latest widespread phase of the batholiths, respectively. Ages of 2716.3 ± 1.4 and 2716 ± 4 Ma for zircons of two minor dioritic phases also fall within this interval. Metamorphic zircons about 2730 and 2715 Ma old from another unit of the batholith indicate a temporal correlation between metamorphism and main plutonic pulses.Sphenes from two samples near the margins of the batholiths yield the same age of 2711 ± 2 Ma as coexisting zircons and date the primary crystallization of the rocks. In contrast, sphenes from six samples from the interior of a batholith yield ages of 2680 ± 10 Ma, which are significantly younger than primary ages of 2950–2716 Ma of coexisting zircons. These sphene ages probably record cooling below about 500 °C during a long and complex cooling process.Several of the studied zircon populations exhibit complex discordance patterns reflecting multiple stages of Pb loss. Zircons in rocks predating the 2700–2730 Ma old metamorphic–plutonic event experienced Pb loss during this event, probably by annealing. A second low-grade event apparently caused chemical alteration of high-U zircon domains and Pb loss about 1750 Ma ago. A late stage of Pb loss affected near-surface zircon domains about 600–0 Ma ago.

Late Archaean—early proterozoic source ages of zircons in rocks from the Paleozoic orogen of western Galicia, NW Spain

U-Pb data are reported for nine suites of zircons and three monazites from the Paleozoic orogen in western Galicia (NW Spain): one paragneiss and six orthogneisses from the early Paleozoic basement, and two Carboniferous (ca. 310 Ma old) intrusions of two-mica granite. New whole-rock Rb-Sr analyses, along with earlier data, indicate an age of ca. 470-440 Ma (Ordovician) for the emplacement of the granitic precursors of the orthogneisses. Monazite from the paragneiss also yields an U-Pb age of ca. 470 Ma. For all nine investigated suites of zircons the U-Pb systematics signal the presence of a minor proportion of Precambrian zircon. The zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3.0 Ga and 2.0 Ga, and lower intercepts between ca. 480 and 460 Ma; the former are interpreted as approaching the age of the old zircon component, and the latter as reflecting either the time of crystallization of new zircons from the magma (orthogneisses), or the time of radiogenic lead loss from the old zircons (paragneiss). The suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470-440 Ma ago, in most cases leading to ‘false’ discordias without geochronological significance. Similarly, the zircons of the two investigated two-mica granites do not produce meaningful discordias because of post-Paleozoic disturbance. The monazite U-Pb systems of the latter granites indicate (sub)recent lead loss. From the high initial 87 Sr 86 Sr ratios an involvement of Precambrian continental crust material is evident in the generation of the early Paleozoic suite of granites, while the zircon U-Pb data give evidence of the presence of about 3.0-2.0 Ga old (late Archaean—early Proterozoic) components in the source material. Zircons from the oldest sedimentary rocks in the area, now present as catazonal paragneisses and a likely source for the granites, likewise reveal a provenance age of 3.0-2.0 Ga. The late Archaean—early Proterozoic source rocks in western Galicia probably formed part of an Archaean continental crust underlying much of western Europe, but mostly modified beyond recognition by younger events.

UPb and RbSr systematics in a polyorogenic segment of the Precambrian shield, central southern Norway

UPb analyses of zircon, monazite and sphene as well as RbSr analyses of whole rocks and minerals have been carried out in an attempt to elucidate the evolution of the Precambrian shield underlying the Caledonian Jotun-nappe in central southern Norway. The earliest event recognized in the area is a high grade metamorphism at 1518 ± 17 m.y. which followed intense magma formation, igneous activity, erosion and sedimentation over a period of maximum 300 m.y. The Sveconorwegian cycle is characterized by the intrusion of two anatectic magma generations at 1014 ± 35 m.y. and 930±10 m.y. that disturbed and partly reset the isotopic systems of the country rocks. A post-magmatic fracturing stage at ∼ 875 m.y. concludes this cycle. RbSr mineral systems were not completely equilibrated in the undeformed shield during the Caledonian event in contrast to new grown minerals in strongly deformed overlying Lower Paleozoic sediments which record the main deformation at 384±18 m.y. Zircon lower intercept ages ranging between 330–370 m.y. show that lead loss of zircon in rocks subjected to low grade metamorphic conditions may be the result of such processes as annealing, alteration, dilation or a combination of them.

Zircon in rocks; 2, Igneous rocks

"This paper attempts to summarize observations on zircon concentrates of igneous rocks from the literature and the writer9s work on southern African and American rocks. The crystallization of zircon is late in the consolidation of basaltic magma, early in that of dioritic or granitic magma, but increasingly delayed in alkaline magmas, apparently in keeping with the rank of the rocks in the alkaline series. In magmatic plutons in which zircon crystallized early, the characters and size of the zircons remain the same throughout. Zircon measurements can thus be used to fingerprint igneous rocks."

Zircons in rocks; Part 1, Sedimentary rocks; Part 2, Igneous rocks

Formulates a series of general rules concerning the size and shape distribution of zircons in sedimentary rocks, and cites the results of studies on the zircon component of Kalahari sands of Bechuanaland, arenites of the Cape system (South Africa), and Permian sediments of the southwestern United States.