L.P. Black

Bio: L.P. Black is an academic researcher. The author has contributed to research in topics: Metamorphism & Recrystallization (geology). The author has an hindex of 1, co-authored 1 publications receiving 1269 citations.

Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon

Abstract: Protolith zircon in high-grade metagranitoids from Queensland, Australia, partially recrystallized during granulite-grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed-system solid-state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re-setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub-grain rotation or local dissolution/re-precipitation, but probably by grain-boundary migration and defect diffusion. The formation of metamorphic zircon by solid-state recrystallization is probably common to high-grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon-derived U–Pb ages and subsequent tectonic models.

The Composition of Zircon and Igneous and Metamorphic Petrogenesis

Abstract: Zircon is the main mineral in the majority of igneous and metamorphic rocks with Zr as an essential structural constituent. It is a host for significant fractions of the whole-rock abundance of U, Th, Hf, and the REE (Sawka 1988, Bea 1996, O’Hara et al. 2001). These elements are important geochemically as process indicators or parent isotopes for age determination. The importance of zircon in crustal evolution studies is underscored by its predominant use in U-Th-Pb geochronology and investigations of the temporal evolution of both the crust and lithospheric mantle. In the past decade an increasing interest in the composition of zircon, trace-elements in particular, has been motivated by the effort to better constrain in situ microprobe-acquired isotopic ages. Electron-beam compositional imaging and isotope-ratio measurement by in situ beam techniques—and the micrometer-scale spatial resolution that is possible—has revealed in many cases that single zircon crystals contain a record of multiple geologic events. Such events can either be zircon-consuming, alteration, or zircon-forming and may be separated in time by millions or billions of years. In many cases, calculated zircon isotopic ages do not coincide with ages of geologic events determined from other minerals or from whole-rock analysis. To interpret the geologic validity and significance of multiple ages, and ages unsupported by independent analysis of other isotopic systems, has been the impetus for most past investigations of zircon composition. Some recent compositional investigations of zircon have not been directly related to geochronology, but to the ability of zircon to influence or record petrogenetic processes in igneous and metamorphic systems. Sedimentary rocks may also contain a significant fraction of zircon. Although authigenic zircon has been reported (Saxena 1966, Baruah et al. 1995, Hower et al. 1999), it appears to be very rare and may in fact be related to …

Atlas of Zircon Textures

Abstract: The mineral zircon is extremely variable both in terms of external morphology and internal textures. These features reflect the geologic history of the mineral, especially the relevant episode(s) of magmatic or metamorphic crystallization (and recrystallization), strain imposed both by external forces and by internal volume expansion caused by metamictization, and chemical alteration. The paper presents a selection of both the most typical, but also of the less common, features seen in zircon, categorized according to the different geological processes responsible for their formation. The atlas is intended as a general guide for the interpretation of zircon characteristics, and of related isotopic data. Zircon has become one of the most widely used minerals for the extraction of information on the prehistory and genesis of magmatic, metamorphic and sedimentary rocks. Much of the geological usefulness of zircon stems from its suitability as a geochronometer based on the decay of U (and Th) to Pb, but in addition it is also the major host of the radiogenic isotopic tracer Hf, and it is used to determine oxygen isotopic compositions and REE and other trace element abundances, all of which yield useful clues concerning the history of the host rock, and in some case, the parent rock in which the precursor zircon crystallized. One of the major advantages of zircon is its ability to survive magmatic, metamorphic and erosional processes that destroy most other common minerals. Zircon-forming events tend to be preserved as distinct structural entities on a pre-existing zircon grain. Because of this ability, quite commonly zircon consists of distinct segments, each preserving a particular period of zircon-formation (or consumption). A long experience and modern instrumentation and techniques have provided the “zircon community” the means to image and interpret preserved textures, and hence to decipher the history and evolution of a rock. One …

Genesis of zircon and its constraints on interpretation of U-Pb age

Abstract: Zircon U-Pb dating is the most commonly used method for isotopic geochronology. However, it has been a difficult issue when relating zircon U-Pb ages to metamorphic conditions in complex metamorphic rocks. Much progress has been made in the past decade with respect to the genesis of zircon and its constraints on interpretation of U-Pb age. Three methods have been proposed to link zircon U-Pb age to metamorphic conditions: (i) internal structure; (ii) trace element feature; (iii) mineral inclusion composition. Magmatic zircon shows typical oscillatory zoning and/or sector zoning, whereas metamorphic zircon has internal structures such as no zoned, weakly zoned, cloudy zoned, sector zoned, planar zoned, and patched zoned ones. Zircons formed in different geological environments generally have characteristic internal structures. Magmatic zircons from different rock types have variable trace element abundances, with a general trend of increasing trace element abundances in zircons from ultramafic through mafic to granitic rocks. Zircons formed under different metamorphic conditions have different trace element characteristics that can be used to relate their formation to metamorphic conditions. It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet. Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions. Therefore, interpretation of zircon U-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on.