Geochemistry and provenance of lower Permian sedimentary rocks, Nambucca Block, southern New England Orogen

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)<b>_N</b> = 6.3) and Th enrichment, Ti, Nb and Ta depletion, and substantial within plate components (Nb/Yb; 0.65–5.16; <i>x</i> = 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; <i>x</i> = 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; <i>x</i> = 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.KEY POINTSMetasediments in the Nambucca Block record geochemical signatures typical of calc-alkaline, continental, arc rocks.Detritus in metasediments was derived from the Keepit arc and Tablelands Complex, southern New England Orogen.Source rocks were subjected to glacial, temperate and humid conditions at different times. Metasediments in the Nambucca Block record geochemical signatures typical of calc-alkaline, continental, arc rocks. Detritus in metasediments was derived from the Keepit arc and Tablelands Complex, southern New England Orogen. Source rocks were subjected to glacial, temperate and humid conditions at different times.

已有大量文献证实，沉积物所记录的地球化学特征，可反映为其提供碎屑物质的母岩的组成与构造背景。楠巴卡地块（Nambucca Block）内的浊积岩（turbidites）便符合这一规律：其物质组成从玄武质（basaltic）到流纹英安质（rhyodacitic）不等，但以安山质（andesitic）至英安质（dacitic）为主。这些浊积岩呈现球粒陨石（chondrite）与正常洋中脊玄武岩（Normal Mid-Ocean Ridge Basalt, N-MORB）的配分模式，具备钙碱性（calc-alkaline）大陆弧岩石的典型特征：轻稀土元素（rare earth elements, REE）的球粒陨石标准化La/Yb比值_N平均值为6.3、钍（Th）富集，钛（Ti）、铌（Nb）和钽（Ta）亏损，且存在显著的板内组分（within plate components），其Nb/Yb比值介于0.65~5.16之间，平均值为3.15。此外，研究区偶见具有洋内岛弧亲缘性（intra-oceanic island arc affinities）的岩石，以及显示构造组构（tectonic fabrics）的长英质火山岩，表明存在来自远端物源的碎屑物质供给。该浊积岩中的大部分碎屑物质源自基皮特弧（Keepit arc）的钙碱性大陆岩系，另有部分来自台地杂岩（Tablelands Complex）的俯冲增生序列。
化学风化指数（Chemical Index of Alteration, CIA）的计算公式为Al₂O₃/(Al₂O₃+Na₂O+K₂O+CaO)×100，其值介于48~83之间（平均值为68），表明这些弧岩系的侵蚀作用曾发生于不同时期的冰川、温带及湿润气候条件下。进一步的化学变异指数值介于0.3~1.85之间（平均值为0.85），揭示了母岩的组成成熟度较高且黏土矿物占比高，指示研究区处于构造抬升微弱、广泛风化作用发育的被动构造背景。
核心要点：
楠巴卡地块内的变沉积岩（metasediments）记录了钙碱性大陆弧岩石典型的地球化学特征。
变沉积岩中的碎屑物质源自新英格兰造山带（New England Orogen）南部的基皮特弧与台地杂岩。
母岩曾先后经历冰川、温带及湿润气候条件下的风化作用。
楠巴卡地块内的变沉积岩记录了钙碱性大陆弧岩石典型的地球化学特征。
变沉积岩中的碎屑物质源自新英格兰造山带南部的基皮特弧与台地杂岩。
母岩曾先后经历冰川、温带及湿润气候条件下的风化作用。