Tracing Garnet Origins in Granitoid Rocks by Oxygen Isotope Analysis: Examples From the South Mountain Batholith, Nova Scotia

Lackey, Jade Star; Erdmann, Saskia; Hark, Jessica S.; Nowak, Robert; Murray, Kendra E.; Clarke, D. Barrie; Valley, John W.

doi:10.3749/canmin.49.2.417

Cited by 32 publications

(13 citation statements)

References 74 publications

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“…The presence of garnet in peraluminous granites, including many in the Helgeland Nappe Complex, is commonplace (e.g., Miller and Stoddard, 1981;Allan and Clarke, 1981;Zen, 1988;Erdmann et al, 2009;Lackey et al, 2011). These studies and many others have shown that such garnets are typically almandine rich, as are those studied here.…”

Section: Discussionsupporting

confidence: 60%

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et al. 2012

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The 465 Ma Svarthopen pluton in northcentral Norway was emplaced under middle-crustal conditions (~700 MPa) into metasedimentary rocks of the Helgeland Nappe Complex. The pluton is characterized by zones of mingling and mixing of gabbro/ diorite with peraluminous, garnet-bearing biotite granite. Variation in bulk-rock Sr and Nd isotope ratios are consistent with simple mixing; however, nonuniform enrichment of Zr and the rare earth elements (REEs) suggests that individual magma batches underwent postmixing fractionation. Hybrid intermediate rocks are characterized by Ca-rich garnet. Such garnet is absent in possible mafi c end members, and garnet in felsic end members is Ca poor. Evidently, the ferroan , peraluminous hybrid rocks promoted garnet stability, and we interpret these garnets to be igneous in origin. Garnets in the hybrids have low Zr contents, positive light REE slopes, and fl at to negative heavy REE slopes with lower REE abundances than in typical igneous garnet. These trace-element data combined with textural evidence suggest that garnet formed near the solidus, after fractionation of zircon, allanite, and possibly xenotime. The Svarthopen pluton is not unique: similar intermediate rocks with Ca-rich garnets crop out adjacent to three other plutons in the region.Formation of garnet-bearing hybrid rocks in the Svarthopen pluton provides an analog for mixing of peraluminous and ferroan endmember magmas in the deep crust, where such mixing should be widespread, particularly in continental arcs and zones of continental collision. Postmixing fractionation of hybrid magmas could greatly increase the diversity of major-and trace-element abundances yet retain an isotopic signature of mixing. More-over, formation of garnet-rich hybrids could result in lower-crustal rocks dense enough to delaminate from the arc crust.

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Section: Discussionsupporting

confidence: 60%

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“…Van Orman et al (2002) showed that the diffusion rates for Sm and Yb are two to three orders of magnitude slower than those of major elements. Several studies (Spear & Kohn, 1996;Van Orman et al, 2002;Vielzeuf et al, 2005;Villaros et al, 2009a;Lackey et al, 2011a) concluded that because the diffusion coefficients are also small for REE, Zr, Y, O, Ca, and other elements analyzed in this study, growth zoning profiles in phenocrystic garnets would require very long times to homogenize, of the order of tens of millions of years. Hence we consider the trace element profiles observed in the Cardigan Pluton garnet to represent growth zoning profiles.…”

Section: Discussion Diffusion Coefficients For Selected Tracmentioning

confidence: 93%

Detecting Peritectic Garnet in the Peraluminous Cardigan Pluton, New Hampshire

Dorais

Tubrett

2011

Journal of Petrology

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The peraluminous Cardigan Pluton of New Hampshire contains restitic garnetites with abundant peritectic garnet that serves as a basis for comparison with garnet in the host-rock. The peritectic garnet is inclusion-rich and contains high concentrations of heavy rare earth elements (HREE), Sc, and Zr and low concentrations of Zn, P, and U. The high HREE, Zr, and Sc result from biotite and zircon melting, buffering liquid compositions. Because the HREE and Sc are compatible in garnet, the peritectic garnet is rich in these elements. Some garnet crystals in the host-rocks have cores with identical inclusions and trace element compositions to the peritectic garnet in the garnetites. We infer that these are also peritectic garnet grains dispersed in the host magma. These cores are mantled by inclusion-poor garnet. These mantles, along with other garnet crystals that are inclusion-poor throughout, show rimward trends of decreasing HREE, Sc, and Zr, and increasing Zn, P, U, and Eu/Eu*.We interpret the trace element variations as records of fractionation as the melt decreased in HREE, Zr, and Sc and was enriched Zn, P, and U. The rimward increase in Eu/Eu* records the effect of feldspar fractionation. Thus the peritectic garnet preserves evidence of crystallization from buffered melts whereas the phenocrystic garnet shows variable trace element abundances indicative of crystallizing from fractionating melts. These differences provide criteria for the recognition of peritectic garnet in the host pluton.

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“…Refractory phases such as garnet and zircon are known to maintain a high degree of inheritance from their sources, even when entrained in granite (e.g., Lackey et al, 2011;Williams, 2001). Table 1 Electron microprobe analyses (wt.%) of experimental granitic melts in equilibrium with common ferromagnesian minerals at 800°C and 200 MPa.…”

Section: Applicationsmentioning

confidence: 99%