The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska

Abstract

Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U–Pb geochronology. However, linking monazite U–Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U–Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~ 550–650 °C and ≥ 3.4 kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U–Pb SHRIMP analysis of the second and third monazite generations yields ages of ~ 55–50 Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle.

Developing an inverted Barrovian sequence; insights from monazite petrochronology

“…Monazite analysis frequently yields complex geochronological datasets (e.g. Buick et al, 2006;Gasser et al, 2012;Hermann and Rubatto, 2003), making it difficult to link the ages with the P -T conditions and crystallisation reactions of the phases. Emerging developments in linking 'age to stage', and particularly the use of combined petrography and traceelement data from major and accessory phases (petrochronology), allow high-spatial precision geochronological data to be more firmly linked to the P -T conditions of accessory phase crystallisation reactions (Foster et al, 2004;Gasser et al, 2012;Janots et al, 2008;Rubatto et al, 2013).…”

“…Buick et al, 2006;Gasser et al, 2012;Hermann and Rubatto, 2003), making it difficult to link the ages with the P -T conditions and crystallisation reactions of the phases. Emerging developments in linking 'age to stage', and particularly the use of combined petrography and traceelement data from major and accessory phases (petrochronology), allow high-spatial precision geochronological data to be more firmly linked to the P -T conditions of accessory phase crystallisation reactions (Foster et al, 2004;Gasser et al, 2012;Janots et al, 2008;Rubatto et al, 2013). This is achieved by detailed observation of textural relationships, geochemical analysis of coexisting accessory phases and systematic documentation of trace-element signatures in major phases that 'fingerprint' monazite-forming reactions (Foster et al, 2002(Foster et al, , 2000(Foster et al, , 2004Gasser et al, 2012;Hermann and Rubatto, 2003;Hoisch et al, 2008;Janots et al, 2006Janots et al, , 2007Janots et al, , 2008Janots et al, , 2009Kingsbury et al, 1993;Pyle and Spear, 2003;Rubatto et al, 2006;Smith and Barreiro, 1990;Spear, 2010;Wing et al, 2003).…”

Speculations Linking Monazite Compositions to Origin: Llallagua Tin Ore Deposit (Bolivia)

“…All spots were pre-ablated for 2 s using a 75 µm spot, 20% laser power, and 10 Hz repletion rate to remove surface contamination (i.e., EPMA carbon coat). The quadrupole time-resolved method involved measurement of 22 analytes at one point per spectral peak, using the integration times of 10 ms ( 31 P and 89 Y), 20 ms ( 23 Na, 27 Al, 47 Ti, 55 Mn, 57 Fe, 75 As, 137 232 Th, and 238 U). The resulting sampling period (0.5062 s) corresponded to >90% detection time, enabling 118 measurements to be made within the dwell interval (60 s), conditions suitable for robust measurement [32].…”

“…Other approaches (Secondary Ion Mass Spectrometry) may be able to detect common Pb, and confirm this hypothesis. Other elements present below detection limits are 23 Na, 27 Al, 47 Ti, 55 Mn, 57 Fe, and 137 Ba. Arsenic-75 is present in the monazite at 195 ± 23 ppm (average of 56 analyses, including uncertainty, ±2σ, Table 3).…”

“…In addition, monazite grains with low U and/or Th are common in rocks that have experienced significant fluid involvement, including authigenic precipitation (e.g., [27,[51][52][53]) or retrograde reactions [54][55][56]. These types of monazites are also found as hydrothermal precipitates [57][58][59] or in carbonatites (e.g., [60][61][62][63]).…”