Relationships Between Tectonics, Volcanism, and Hydrothermal Venting in the New Hebrides and Mariana Back-Arc Basins, Western Pacific
Creators: name: Anderson, Melissa
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2018
Subjects: subject: Massive sulfide
subject: Back-arc
subject: New Hebrides
subject: Mariana
subject: Hydrothermal venting
subject: Geology
subject: FOS: Earth and related environmental sciences

description: Understanding the controls on the distribution and type of hydrothermal venting in modern oceanic spreading environments is key to developing tools for exploration and understanding the metallogeny of ancient massive sulfide deposits. Compared to mid-ocean ridges, subduction zones are characterized by additional tectonic complexities, including arc-ridge collisions, arc rotations, pre-existing structures, and variable distances to the arc. This thesis addresses the question, “How do tectonic complexities associated with subduction influence the structure and volcanic evolution of a back-arc basin, and how do they affect the distribution and type of hydrothermal venting?” A multi-scaled approach was used to address this question in the nascent back-arc region of the New Hebrides and in the more advanced stages of opening of the Mariana back-arc basin. In the New Hebrides, an arc-ridge collision segmented the volcanic front and affected the southern and northern back-arc regions in different ways. In the southern Coriolis Troughs (CT), voluminous eruptions are closely linked to the ridge collision, forming a large shield volcano in the near-arc region (Nifonea Volcano). The caldera-hosted eruptions produced high-temperature but short-lived magmatic-hydrothermal activity restricted to the shield volcano. In the northern Jean Charcot Troughs (JCT), ridge collision caused a reversal in the rotation of the arc, reducing extension in the south and increasing extension in the north. Unlike the CT, extension in the JCT is strongly affected by pre-existing structures, which form irregular widely-spaced grabens and volcanic ridges and magmatism in the central part of the back-arc. Here, hydrothermal venting is focused along deeply penetrating faults, associated with widespread tectonic extension. Detailed studies of the mineralogy and geochemistry of the ore and alteration at the Tinakula deposit reveal that massive sulfide accumulation in the region dominated by tectonic extension is characterized by longer-lived, lower-temperature venting than at Nifonea. Hydrothermal activity in the JCT at Tinakula is dominated by (1) long-lived heat from an underlying magma source; (2) fluid circulation along a fissure with long-lived or reactivated permeability; (3) enrichment in fluid-mobile elements such as Ba that are transported at low temperature; (4) mixing of cold seawater with hydrothermal fluids within the permeable volcaniclastic substrate and at the seafloor; (5) water depth controls on maximum hydrothermal vent temperatures; and (6) reduced permeability of the host volcaniclastic succession at the site of mineralization caused by precipitation of alteration minerals and sulfates, focusing fluid flow. The different styles of volcanic and hydrothermal activity closely resemble those of mid-ocean ridge environments in areas that are dominated by tectonic rather than magmatic extension. A comparison with the more advanced stages of rifting and segmentation of the Mariana back-arc demonstrates that Mid-Ocean Ridge (MOR)-type structural and magmatic controls on hydrothermal activity are important during all stages of back-arc basin evolution. This work highlights the diversity of volcanic eruption styles and hydrothermal venting from the earliest stages of back-arc rifting to the advanced stages of basin opening and shows that processes normally associated with MOR-type spreading are directly analogous to back-arc basin systems. However, additional tectonic complexities (e.g., ridge-arc collisions) have a major impact on the location and type of magmatic and hydrothermal activity at back-arc spreading centers, with important implications for understanding ancient volcanic-hosted massive sulfide deposits that mainly formed in back-arc basins.