Abstract

It is well documented that sediments record geochemical signatures that reflect the composition and tectonic setting of the rocks providing the detritus for them. This is true of the turbidites in the Nambucca Block that record compositions that vary from basaltic to rhyodacitic but are dominantly andesitic to dacitic. They show chondrite and N-MORB patterns, characteristic of calc-alkaline, continental arc rocks, namely light REE (av. La/Yb)_N = 6.3) and Th enrichment, Ti, Nb and Ta depletion, and substantial within plate components (Nb/Yb; 0.65–5.16; x = 3.15). Rare occurrences of rocks with intra-oceanic island arc affinities and felsic volcanic rocks showing tectonic fabrics occur, indicating more distal sources contributed detritus. Most detritus in the turbidites has been derived from the calc-alkaline, continental rocks of the Keepit arc, some from the subduction–accretion sequences of the Tablelands Complex. CIA [Al₂O₃/(Al₂O₃+Na₂O + K₂O + CaO)×100] values (48–83; x = 68) indicate that erosion of these arc sequences occurred during glacial, temperate and humid conditions at different times. Further, index of chemical variability values (0.3–1.85; x = 0.85) reveal a compositional maturity and a high percentage of clays in the source rocks suggesting a passive tectonic setting in which minimal uplift and extensive weathering took place.

1. KEY POINTS

2. Metasediments in the Nambucca Block record geochemical signatures typical of calc-alkaline, continental, arc rocks.

3. Detritus in metasediments was derived from the Keepit arc and Tablelands Complex, southern New England Orogen.

4. Source rocks were subjected to glacial, temperate and humid conditions at different times.

摘要

有据可查的是，沉积物记录了地球化学特征，这些特征反映了为它们提供碎屑的岩石的组成和构造环境。Nambucca 地块中的浊积岩也是如此，这些浊积岩记录的成分从玄武岩到流纹岩不等，但主要是安山岩到英安岩。它们显示球粒陨石和 N-MORB 模式，具有钙碱性大陆弧岩石的特征，即轻稀土 (av. La/Yb) _N = 6.3) 和 Th 富集，Ti、Nb 和 Ta 贫化，以及大量的板块内成分（ Nb/Yb；0.65–5.16；x = 3.15）。罕见的具有洋内岛弧亲缘关系的岩石和显示构造结构的长英质火山岩出现，表明更多的远端来源贡献了碎屑。浊积岩中的大多数碎屑来自 Keepit 弧的钙碱性大陆岩石，一些来自 Tablelands 复合体的俯冲-增生序列。CIA [Al ₂ O ₃ /(Al ₂ O ₃ +Na ₂ O + K ₂ O + CaO)×100] 值 (48–83; x  = 68) 表明这些弧序列的侵蚀发生在冰川期、温带期和潮湿期不同时期的条件。此外，化学变异值指数 (0.3–1.85; x = 0.85) 揭示了烃源岩中的成分成熟度和高百分比的粘土，表明处于被动构造环境，其中发生了最小的隆起和广泛的风化。

1. 关键点

2. Nambucca Block 中的变质沉积物记录了典型的钙碱性、大陆性、弧形岩石的地球化学特征。

3. 变质沉积物中的碎屑来自新英格兰造山带南部的 Keepit 弧和 Tablelands Complex。

4. 烃源岩在不同时期受到冰川、温带和潮湿条件的影响。