Cretaceous metamorphism, magmatism and shearing in the Waipuna Valley, directly south of the Reefton Goldfield

Rocks in the Waipuna Valley in Westland show that the area immediately south of the Reefton Goldfield has been dramatically affected by Cretaceous tectonism. The main lithology in the Waipuna Valley is metasedimentary Greenland Group that is here subdivided into four types (I, II, III and IV). Type I contains abundant detrital grains with a weak Palaeozoic greenschist facies overprint and is compositionally indistinguishable from unmineralised Greenland Group within the goldfield. Type II contains detrital grains, but also a weak amphibolite facies foliation or, where proximal to granitoid intrusions, a hornfelsic texture. Type III contains no detrital components and records polyphase recrystallisation. U–Th–Pb monazite dating indicates that metamorphism in the Type III rocks occurred, at least in part, at 108.1 ± 1.2 Ma. Type IV Greenland Group has partially melted. Zircon from granitoids intruding Greenland Group in the Waipuna Valley indicate Cretaceous (c. 111 Ma) emplacement. Greenland Group Types III and IV, and the granitic plutons, have been variably sheared with quartz textures indicating progressive cooling during deformation. A mylonite zone occurs just above outcrop of the Type IV rocks. Field and age relationships are interpreted to show that the Waipuna Valley exposes portions of an upper plate and lower plate to a Cretaceous ductile shear zone that is here named the Waipuna Shear Zone. The overall structure is similar to the nearby Cretaceous extensional Paparoa Metamorphic Core Complex. The Reefton Goldfield lies in the upper plate to the Waipuna Shear Zone, not far above the mylonite zone. Cretaceous metamorphism and shearing of Greenland Group could have aided formation and then transport of gold-bearing fluids into structurally higher rocks.

西地（Westland）怀普纳谷（Waipuna Valley）的岩石露头显示，里夫顿金矿区（Reefton Goldfield）南侧紧邻区域受到了白垩纪构造作用的强烈改造。怀普纳谷的主要岩性为变质沉积岩格林兰群（Greenland Group），本次研究将其划分为I、II、III、IV四个类型。I型岩石富含碎屑颗粒，仅受弱古生代绿片岩相叠加改造，其地球化学组成与金矿区内未矿化的格林兰群难以区分。II型岩石既含有碎屑颗粒，又发育弱角闪岩相面理；在紧邻花岗岩类侵入体（granitoid intrusions）的区段，还可见角岩结构。III型岩石不含碎屑组分，记录了多期重结晶作用。通过U-Th-Pb独居石定年（U–Th–Pb monazite dating）分析可知，III型岩石的变质作用至少有一期发生于108.1 ± 1.2 Ma。IV型格林兰群发生了部分熔融。侵入怀普纳谷格林兰群的花岗岩类岩体的锆石（zircon）定年结果显示，其侵位时代为白垩纪（约111 Ma）。III型、IV型格林兰群以及花岗岩岩体均遭受了不同程度的剪切变形，其石英结构特征表明变形过程中岩体持续降温。IV型岩石露头上方紧邻糜棱岩带（mylonite zone）。野外地质与年代学关系综合分析表明，怀普纳谷出露了白垩纪韧性剪切带（ductile shear zone）的上盘与下盘部分区段，本次将该剪切带命名为怀普纳剪切带（Waipuna Shear Zone）。该构造整体特征与附近的白垩纪伸展型帕帕罗阿变质核杂岩（Paparoa Metamorphic Core Complex）相似。里夫顿金矿区位于怀普纳剪切带上盘，距离糜棱岩带不远。格林兰群经历的白垩纪变质与剪切作用，可能促进了含金流体的形成，并将其运移至构造位置更高的岩石中。