High-T–low-P thermal anomalies superposed on biotite-grade rocks, Wongwibinda Metamorphic Complex, southern New England Orogen, Australia: heat advection by aqueous fluid?

Abstract

The Wongwibinda Metamorphic Complex is characterised by ∼400 km² of biotite-grade rocks with irregular, km-scale zones of cordierite-bearing high-T rock (T > 550°C). Cordierite-bearing rocks include different textures: (i) cordierite–K-feldspar–spotted hornfels; (ii) sheared cordierite–K-feldspar–augen schists; and (iii) migmatites with or without comparatively coarse garnet poikiloblasts. Integrated petrography, mineral chemistry and mineral equilibria modelling indicate metamorphic peak conditions in samples of hornfels of approximately P ≤ 250 MPa and T = 570–620°C. Two samples of sheared rock preserve peak conditions of approximately T = 670–690°C or T < 600°C and P ≤ 200 MPa, consistent with rapid cooling or variable thermal structure in the Glen Mohr Shear Zone. Electron microprobe U–Th–Pb monazite data indicate a date of 291.5 ± 1.8 Ma for the shear zone, indistinguishable from the age of Hillgrove Plutonic Suite granitoids in the complex. Monazite ages are complex in the hornfels samples that were likely, on the basis of structural relationships, metamorphosed early in the history of the Wongwibinda Metamorphic Complex (ca 300 Ma). Previous monazite dating of migmatite samples (ca 297 Ma) suggests that the metamorphic cycle was short-lived, lasting less than ca 10 m.y. Metamorphic field gradients are <15–23°C km⁻¹ across much of the complex but locally steep (>50–100°C km⁻¹) around the high-T rocks. Rejection of most heat sources leaves regional conductive heating as the most plausible explanation for the smooth, broad and shallow metamorphic field gradient in the biotite-grade rocks. Another mechanism of heat transfer is required to explain the local, steep metamorphic field gradients and development of high-T cordierite-bearing rocks. The spatial association of quartzite units centred within two cordierite-bearing high-T domains and an increased abundance of quartz veins above the cordierite isograd suggests heat advection by aqueous fluid locally perturbed the broad conductive heating. Fluid was channelled within shear zones and locally infiltrated nearby rocks. Variable fluid flux and strain have produced the range of different textures in cordierite-bearing rocks of the Wongwibinda Metamorphic Complex.

Wongwibinda 变质复合体的特征是，约400 平方公里的黑云母级岩石带有不规则的公里级数的含堇青石高温岩石（T>550˚C）。含堇青石岩石包括不同的结构：（i）点状堇青石-K-长石角石；（ii）剪切堇青石-K-长石-眼斑片岩；（iii）带有或不带较粗石榴石变嵌晶的混合岩。岩石学、矿物化学和矿物均衡模式的综合研究，发现角岩内变质高峰条件大约是P≤250MP及T=570-620˚C。两个剪切岩石标本保存的高峰条件为T=670-690˚C或T<600˚C及P≤200MPa，与Glen Mohr 剪切带中的快速冷却或可变热结构相一致。电子微探U-Th-Pb独居石资料将剪切岩石的年龄确定为291.5±1.8Ma，与复合体中的Hillgrove深成岩套花岗岩的年龄没有区别。角岩标本中的独居石年龄是复杂的。根据构造关系，角岩可能在Wongwibinda变质复合体的早期历史中就已变质了（约300Ma）。以前对混合岩标本（约297Ma）中的独居石的年龄确定说明变质周期很短，不到约一千万年。复合体很多部分的变质场梯度是<15-23˚CKm-1，但在高温岩石周围却有局部性的陡梯度（>50-100˚CKm^-1）。 对黑云母级岩石中的光顺、宽阔且浅的变质场梯度最可接受的解释是，大部分热源的废热离开区域性传导加热。要解释局部性的陡变质场梯度和高温含堇青石岩石的形成，必须依靠另一种热传递机制。位于两个含堇青石的高温区域中部的石英岩的空间关系及堇青石等变质线之上的石英脉丰富度的增加，说明通过水流的热平流作用局部搅扰了宽传导性加热。流体在剪切带内流动并局部渗透附近的岩石。Wongwibinda变质复合体的含堇青石岩石内流体流动和应力的变化导致了一系列不同的结构。

KEY WORDS:

• monazite
• pseudosection
• cordierite
• meta-turbidite
• migmatite
• HTLP metamorphism
• Wongwibinda Metamorphic Complex
• New England

关键词:

• 独居石
• 假剖面
• 堇青石
• 变-浊积岩
• 混合岩
• HTLP变质作用
• Wongwibinda变质复合体
• 新英格兰

ACKNOWLEDGEMENTS

Macquarie University Research Development Grant funding to NRD provided financial support to conduct this research. We thank the local landowners for permission to visit and sample localities in the Wongwibinda Metamorphic Complex. I. Wainwright (University of New South Wales) performed XRF analyses. K. Goemann is thanked for help with the monazite geochronology at UTAS. Catherine Stuart and Kim Jessop provided the photomicrographs for Figure 3 (e, f). A critical review by an anonymous reviewer was helpful. The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners and Macquarie University. This is contribution 887 from the Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents (http://www.gemoc.mq.edu.au).

Notes

Loss of ignition (LOI) at 1050°C. A less than symbol (<) represents concentrations below the stated lower limit of detection.

Fe²⁺ is total Fe.

pin., cordierite is completely pseudomorphed by pinnite.

σ fit, goodness of fit parameter (√MSWD).