Generation of granitic magmas in the lower crust: a natural example from Mt Daniel, Fiordland, New-Zealand

Bhattacharya, Shrema (2010) Generation of granitic magmas in the lower crust: a natural example from Mt Daniel, Fiordland, New-Zealand. PhD thesis, James Cook University.

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The base of the 10-20 km thick, batholithic, Western Fiordland Orthogneiss (WFO) is exposed in northern Fiordland as the Mount Daniel Complex (MDC). It forms a ~100-metre thick, sheeted complex between the WFO and the underlying dominantly dioritic basement of Arthur River Complex (ARC). Although previously called a migmatite aureole at the WFO base, field relations show it formed after the WFO was solidified and injected by numerous rectilinear trondhjemitic dykes. U-Pb zircon analysis confirms emplacement of the WFO at 125 Ma, with MDC formation slightly later, between 120-112 Ma. Successive dyke injection and sheeting during tight to isoclinal folding, and fold asymmetry, indicates that the MDC was emplaced syn-tectonically into an active, sinistral reverse shear zone associated with exhumation of the WFO. The MDC is composed almost entirely of intrusive metre-scale sheets, ranging in composition from mafic dykes, low-Na and high-Na intermediate (trondhjemitic) dykes, high-K granitic dykes and the dominant intermediate-composition Mt Daniel Sheets (MDS) of the complex 55-65% SiO2. The compositional range of MDC vary from 44wt% to 76 wt% SiO2, and has a typically sodic character (Na2O 4-8 wt %). With the exception of very sodic trondhjemitic dykes (Na2O >6 wt%), the MDC is similar to coeval high-level granitic plutons of the Separation Point Batholith (SPB), located northward from the study area. WFO shows a different geochemical trend from the other MDC rocks and a more restricted isotopic composition. This data and field evidence suggests that the WFO was not involved in formation of the MDC.

The wide spectrum of MDC rock types requires multiple sources. Flat, MORB-like REE patterns of the mafic dykes indicate derivation from spinel lherzolite (< 70 Km depth). All other rock types come from crustal sources. The trondhjemitic dykes have variably depleted HREE and minor Eu anomalies compared to the other rock-types and high Sr/Y ratios (>40) indicating their derivation from a deep mafic crustal source ≥13 Kb where garnet was stable and plagioclase unstable. The high-K granites have low Rb content, suggesting residual biotite in the source rock. It is argued, based on published experimental data, that the high-K magmas were hydrous and formed near the granite solidus at ~10 kbar.

Bimodal (felsic and mafic) magmas dominate the lower section of the MDC, but more homogenized intermediate dioritic magmas dominate the upper section. Mixing of the magmas occurred by stirring during intrusion into the active shear zone. Trondhjemitic dyke injection was continuous throughout MDC formation. The isotopic εNd-initial Sr wholerock data and Hf isotopic data of zircon from different rock types show a crust-mantle mixing array, and the coherent major and trace element variation within the MDC is broadly consistent with mixing, rather than fractionation. Mixing of the (high- and low-Na) intermediate dyke magmas with the mafic and granitic dyke magmas can explain the chemical and isotopic variation within the MDC, including the REE and multielement variation patterns.

An alternative explanation for the chemical diversity of the MDC is that bulk assimilation of ARC basement has contributed a major component. Assimilation is most clearly observed in the lower sections of the MDC, but less so in the upper homogenized sections. However, the presence of inherited zircon of Carboniferous age in most samples indicates it was pervasive. The similarities in the Hf isotopic composition of zircons from trondhjemitic dykes and SPB granites with the ARC (136-129 Ma) and Darran Complex (143-136 Ma) suggest they represent the dominant juvenile mafic crustal source or equivalent. Few inherited Mesozoic zircons exist in the MDC, but this probably reflects the lack of zircon in the inferred primitive (gabbroic?) protolith. Evidence for an older evolved source component comes from Carboniferous-aged zircons, which are also present in the ARC. This is probably the major source of the high-K granitic magmas. Thus, this study proposes crustal-assimilation and mixing of four major source components in the lower crust produced the chemical diversity of the MDC.

The short time span (120-112 Ma) represented by MDC rocks, the simultaneous presence of Early Cretaceous magmatic and metamorphic zircons, even within the same rock, and the presence of both HREE-depleted and non-depleted rims zircon in granites, suggests a very dynamic transitional magmatic-metamorphic environment associated with the MDC formation. The magmatic to metamorphic condition was associated with a change from open to closed system chemical processes.

The formation of trondhjemitic dykes at ~13 Kbar depth after the burial of WFO at ~120 Ma and formation of granites at comparatively lower depth at ~10 Kb (between 120-112 Ma) is consistent with an exhumation path for the MDC at this stage. Thus, the MDC represents the retrograde part of an anticlockwise P-T-t path. Exhumation could have been associated with crustal extension, for which limited evidence exists regionally, or it can be associated with thrusting. If the latter, the crust must have been undergoing rapid erosion during crustal shortening. The MDC represents a rare exposed example of magma generation in the lower crust. The critical aspect for preservation was the presence of hydrous granitic magmas that froze as the Mt Daniel shear zone crossed the saturated granite solidus. Normally, more anhydrous magmas, possibly also hotter because of mafic input, would move out of the source region and lose the evidence for the incremental granite generation process by sheet/dyke focussing along active shear zones, assimilation and mixing.

The MDC rock types cover the compositional spectrum of the upper crustal SPB, including its sodic, HiSrY character. However, the MDS spectrum reflects mixing whereas the SPB reflects fractionation from relatively homogeneous parent magma. The evidence from the MDC suggests that HiSrY parental, intermediate-composition magmas are produced by large-scale bulk assimilation of the pre-existing lower crust, itself of sodic character, with more sodic dyke material mixed in. The blended magmas segregate and coalesce in upper crustal chambers, where virtually all evidence of the magma generation process is lost.

Item ID: 19040
Item Type: Thesis (PhD)
Keywords: granite, Fiordland Mt Daniel, zircon, Hf-isotope, granitic magmas, New Zealand, orogenesis, petrology, crust formation, geochemistry, zircon morphology, petrogenesis, isotope geochemistry
Date Deposited: 28 Nov 2011 22:59
FoR Codes: 04 EARTH SCIENCES > 0402 Geochemistry > 040203 Isotope Geochemistry @ 50%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 50%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840107 Titanium Minerals, Zircon, and Rare Earth Metal Ore (e.g. Monazite) Exploration @ 100%
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