Provenance of Cambrian conglomerates from New Zealand: implications for the tectonomagmatic evolution of the SE Gondwana margin

The oldest rocks in New Zealand are the Mid- to Late Cambrian intra-oceanic island arc rocks of the Takaka terrane (Devil River arc). The provenance of Cambrian conglomerates stratigraphically above the exposed arc succession was studied to constrain the late stages of arc evolution and its accretion to continental crust. The Dead Goat Conglomerate contains two distinct groups of igneous clasts: (1) intermediate to felsic volcanic clasts with moderately enriched light rare earth element (LREE) and high field strength element (HFSE) contents and positive ϵNd₅₀₀ (+2.1) that were derived from a medium-K calc-alkaline source, probably the main sequence of the Devil River arc; (2) dioritic to metagranitic plutonic clasts strongly enriched in LREE and HFSE and with ϵNd₅₀₀ of +3.5 to +5.9 that were derived from a high-K arc source, probably the uppermost units of the Devil River arc. This is consistent with a new U–Pb sensitive high-resolution ion microprobe age of 496 ± 6 Ma. The Lockett Conglomerate also contains two distinct groups of igneous clasts: (1) ultramafic to intermediate igneous clasts identified as boninitic to transitional low-K calc-alkaline arc-related rocks based on depleted REE and HFSE abundances; (2) ‘I’-type metagranitoid clasts derived from a distinct Andean type continental margin, as indicated by ϵNd₅₀₀ as low as −7.1. Both conglomerates contain sandstone clasts derived from a common old, multi-cycle continental source with ϵNd₅₀₀ of −14.2 to −15.7, and no suitable source has been found in present-day New Zealand. The new provenance data from these conglomerates constrain the time of accretion of the Devil River arc to the palaeo-Pacific Gondwana margin and provide new information on the structural evolution of the accretionary event.

新西兰最古老的岩石为塔卡卡地体（Takaka terrane）的中晚寒武世洋内岛弧岩系，即魔鬼河弧（Devil River arc）。本次研究针对出露弧层序之上的寒武纪砾岩开展物源分析，以约束该弧演化的晚期阶段及其向大陆地壳的增生过程。死山羊砾岩（Dead Goat Conglomerate）发育两类截然不同的火成岩碎屑：其一为中酸性火山岩碎屑，其轻稀土元素（LREE）与高场强元素（HFSE）含量中等富集，εNd₅₀₀值为+2.1（正异常），源自中钾钙碱性物源区，大概率对应魔鬼河弧的主体层序；其二为闪长质至变花岗质深成岩碎屑，其轻稀土元素与高场强元素强烈富集，εNd₅₀₀值介于+3.5至+5.9之间，源自高钾弧物源区，推测对应魔鬼河弧的最上部层位。该结果与新获得的铀铅（U-Pb）高灵敏度高分辨率离子探针测年结果（496 ± 6 Ma）一致。洛基特砾岩（Lockett Conglomerate）同样发育两类截然不同的火成岩碎屑：其一为超基性至中性火成岩碎屑，基于其稀土元素与高场强元素亏损的特征，可将其归为玻安岩质至过渡性低钾钙碱性弧相关岩石；其二为‘I’型变花岗质碎屑，其εNd₅₀₀值低至-7.1，指示其源自特征鲜明的安第斯型大陆边缘物源区。两类砾岩均含有源自同一古老多旋回大陆物源区的砂岩碎屑，其εNd₅₀₀值介于-14.2至-15.7之间，但现今新西兰境内尚未发现与之匹配的物源区。本次获得的两类砾岩物源数据，限定了魔鬼河弧向古太平洋冈瓦纳大陆边缘增生的时间，并为该增生事件的构造演化提供了新的认知。