### REE-minerals

Monazite, xenotime, and allanite are REE1-minerals sensu stricto because lanthanides (La…Lu) and yttrium are critical constituents in them Apatite does not require REE, but because it contains substantial REE in many rocks, it is included in this review All four minerals also host unusually high radionuclide concentrations, notably Th and U, forming the basis of their utility as geochronometers

This quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic ( P–T ) conditions so can lend petrogenetic context to chronology based on other minerals Part I of this review assembles the basic requisites prior to integrative petrochronologic analysis Individual characteristics of the four REE-minerals are addressed first, ie, their crystal chemistry and stability relations Thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next, and finally their chronology is summarized Part II presents case studies to highlight the specific strengths of REE-minerals used to resolve the dynamics of a broad range of processes, from diagenetic to magmatic conditions Finally, a brief section at the end outlines a few of the current challenges and promising perspectives for future work

To introduce the four REE-minerals in style, let us recall the origins of their names The three phosphates have well respected Greek grandparents, and allanite has solid Scottish roots, yet of all four of them show idiosyncracies in etymology or type material

Apatite had long puzzled naturalists, as it shows great chemical and physical variability and can resemble other minerals Once properly identified, Abraham Gottlieb Werner named it apatite His reasoning referred to the Greek root ἀπατὰω and giving the precise Latin translation: decipio  Taken literally, both mean “I deceive” or “I mislead”, which sounds like an apt confession from this mineral for having fooled …

Petrochronology Based on REE-Minerals: Monazite, Allanite, Xenotime, Apatite

At the core of petrochronology is the relationship between geochronology and the petrological evolution of major mineral assemblages. The focus of this chapter is on outlining some of the available strategies to link inferred reaction sequences and microstructures in metamorphic rocks to the ages obtained from geochronology of accessory minerals and datable major minerals. Reaction sequences and mineral assemblages in metamorphic rocks are primarily a function of pressure ( P ), temperature ( T ) and bulk composition ( X ). Several of the major rock-forming minerals are particularly sensitive to changes in P–T (e.g., garnet, staurolite, biotite, plagioclase), but their direct geochronology is challenging and in many cases not currently possible. One exception is garnet, which can be dated using Sm–Nd and Lu–Hf geochronology (e.g., Baxter et al. 2013). Accessory mineral chronometers such as zircon, monazite, xenotime, titanite and rutile are stable over a relatively wide range of P–T conditions and can incorporate enough U and/or Th to be dated using U–Th–Pb geochronology. Therefore, linking the growth of P–T sensitive major minerals to accessory and/or major mineral chronometers is essential for determining a metamorphic P–T–t history, which is itself critical for understanding metamorphic rocks and the geodynamic processes that produce them (e.g., England and Thompson 1984; McClelland and Lapen 2013; Brown 2014).

Linking the ages obtained from accessory and major minerals with the growth and breakdown of the important P–T sensitive minerals requires an understanding of the metamorphic reaction sequences for a particular bulk rock composition along a well-constrained P–T evolution. Fortunately, the phase relations and reaction sequences for the most widely studied metamorphic protoliths (e.g., pelites, greywackes, basalts) can be determined using quantitative phase equilibria forward modelling (e.g., Powell and Holland 2008). Comprehensive activity–composition models of the major metamorphic minerals in large chemical systems (e.g., White et al. 2014a) allow …

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Phase Relations, Reaction Sequences and Petrochronology

Mass transport and fractionation during monazite alteration by anisotropic replacement

Granulites of the southern East African Orogen formed by continental collision during Gondwana assembly. The highest metamorphic gradients of 25–50 °C km−1 were attained at 0.58–0.53 Ga in a microcontinental block that was sandwiched between two collisional sutures and is now exposed in Madagascar and southern India. The 50 Myr duration of extreme pressure–temperature (P–T) conditions and lack of coeval mantle magmatism suggest that metamorphism was driven by radiogenic heat accumulation beneath a long–lived orogenic plateau. Bounding sutures most likely record transfer of this microcontinent across the Neoproterozoic Mozambique Ocean, analogous to Gondwanan terranes that crossed Tethys before final India–Asia collision and, like Tibet, these sutures mark the edges of a plateau that formed following terminal ocean closure and collision. Both sutures record moderate metamorphic gradients of 15–25 °C km−1 but with quite different ages. Metamorphism along the western suture at 0.65–0.61 Ga followed the end of magmatism in an adjacent 0.85–0.65 Ga ocean–arc terrane. It has an anti–clockwise P–T path that reflects preferential thickening of the hot arc during early stages of collision, and dates ocean closure at the western suture. Metamorphism along the eastern suture at 0.53–0.51 Ga has a clockwise P–T path and is widely assumed to date terminal collision in the East African Orogen. However, this event was coeval with rapid exhumation of granulites in the adjacent plateau and is more likely to reflect reactivation of a much older eastern suture during plateau collapse. Great care should be taken when using metamorphism to date ocean closure in ancient orogens. Rocks with hot metamorphic gradients give poor age constraints on initial collision because peak T is attained >50 Myr after ocean closure if radioactivity is a major part of the heat budget. Suture zone rocks with moderate metamorphic gradients can provide more reliable estimates for the time of ocean closure but are also prone to later reactivation in orogens with protracted histories.

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

In-situ monazite Th–U–total Pb dating and zircon LA–ICP–MS depth-profiling was applied to metasedimentary rocks from the Vaimok Lens in the Seve Nappe Complex (SNC), Scandinavian Caledonides. Results of monazite Th–U–total Pb dating, coupled with major and trace element mapping of monazite, revealed 603 ± 16 Ma Neoproterozoic cores surrounded by rims that formed at 498 ± 10 Ma. Monazite rim formation was facilitated via dissolution–reprecipitation of Neoproterozoic monazite. The monazite rims record garnet growth as they are depleted in Y2O3 with respect to the Neoproterozoic cores. Rims are also characterized by relatively high SrO with respect to the cores. Results of the zircon depth-profiling revealed igneous zircon cores with crystallization ages typical for SNC metasediments. Multiple zircon grains also exhibit rims formed by dissolution–reprecipitation that are defined by enrichment of light rare earth elements, U, Th, P, ± Y, and ± Sr. Rims also have subdued Eu anomalies (Eu/Eu* ≈ 0.6–1.2) with respect to the cores. The age of zircon rim formation was calculated from three metasedimentary rocks: 480 ± 22 Ma; 475 ± 26 Ma; and 479 ± 38 Ma. These results show that both monazite and zircon experienced dissolution–reprecipitation under high-pressure conditions. Caledonian monazite formed coeval with garnet growth during subduction of the Vaimok Lens, whereas zircon rim formation coincided with monazite breakdown to apatite, allanite and clinozoisite during initial exhumation.

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High-spatial resolution dating of monazite and zircon reveals the timing of subduction–exhumation of the Vaimok Lens in the Seve Nappe Complex (Scandinavian Caledonides)

S.W. Kim

Papers

Linking monazite geochronology with fluid infiltration and metamorphic histories: Nature and experiment