Extraction, Storage and Eruption of Multiple Isolated Magma Batches in the Paired Mamaku and Ohakuri Eruption, Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ) is well known for its extraordinary rate of rhyolitic magma generation and caldera-forming eruptions. Less is known about how large volumes of rhyolitic magma are extracted and stored prior to eruption, and the role tectonics might play in the process of melt extraction and control of caldera eruption(s). Here we present a new model for the extraction, storage and simultaneous eruption of the >245 km3 paired Mamaku and Ohakuri magmas sourced from calderas centred ∼30 km apart (the Rotorua and Ohakuri calderas, respectively) in the central TVZ. The Mamaku and Ohakuri ignimbrites share a similar bulk pumice composition and the same phenocryst assemblage; however, bulk-rock compositions suggest several poorly mixed magma types in each erupted volume, which are randomly distributed throughout the eruptive deposits. To refine models of the pre-eruptive geometry of the magmatic system and discuss a possible origin for triggering of each eruption, we present an expanded database of matrix glass and quartz-hosted melt inclusion compositions along with the existing bulk-rock and mineral compositions. Major and trace element compositions show that the region produced five different magma batches, extracted from the same source region, and a continuous intermediate mush zone beneath the Mamaku-Ohakuri region is suggested here. These magma batches were most probably juxtaposed but isolated from each other in the upper crust, and evolved separately until eruption. The observed geochemical differences between the batches are likely to be generated by different extraction conditions of the rhyolitic melt from a slightly heterogeneous mush. The lack of evidence for more mafic recharge prior to eruption (for example, there are no bright cathodoluminescence rims on quartz crystals) suggests that a magmatic input is unlikely to be an eruption trigger. However, tectonic activity could be an efficient way to trigger the eruption of isolated magma batches, with the evacuation of one magma batch causing a disturbance to the local stress field and activating regionally linked faults, which then lead to the eruption of additional magma batches and associated caldera subsidence. In addition, the extensional tectonic regime coupled with a high heat flux could be the controlling factor in the emplacement of some of the shallowest and most SiO2-rich magmas on Eart

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

Chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic–hydrothermal systems such as the Coroccohuayco Fe–Cu–Au porphyry–skarn deposit, Peru. Here, we used recent partitioning models to determine the Cl and S concentration of the melts from the Coroccohuayco magmatic suite using apatite and amphibole chemical analyses. The pre-mineralization gabbrodiorite complex hosts S-poor apatite, while the syn- and post-ore dacitic porphyries host S-rich apatite. Our apatite data on the Coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity (from the gabbrodiorite complex to the porphyries) causing the dominant sulfur species to shift from S2− to S6+ at upper crustal pressure where the magmas were emplaced. We suggest that this change in sulfur speciation could have favored S degassing, rather than its sequestration in magmatic sulfides. Using available partitioning models for apatite from the porphyries, pre-degassing S melt concentration was 20–200 ppm. Estimates of absolute magmatic Cl concentrations using amphibole and apatite gave highly contrasting results. Cl melt concentrations obtained from apatite (0.60 wt% for the gabbrodiorite complex; 0.2–0.3 wt% for the porphyries) seems much more reasonable than those obtained from amphibole which are very low (0.37 wt% for the gabbrodiorite complex; 0.10 wt% for the porphyries). In turn, relative variations of the Cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the Coroccohuayco magmatic suite. This confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement, and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100–200 MPa. Finally, mass balance constraints on S, Cu and Cl were used to estimate the minimum volume of magma required to form the Coroccohuayco deposit. These three estimates are remarkably consistent among each other (ca. 100 km3) and suggest that the Cl melt concentration is at least as critical as that of Cu and S to form an economic mineralization

Stratigraphy and structure of the Ngatamariki geothermal system from new zircon U-Pb geochronology: Implications for Taupo Volcanic Zone evolution

Recent drilling at the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand has provided new constraints on the stratigraphy and volcanic evolution of the region. Over 2800m thickness of volcanic products are present at Ngatamariki, mainly comprised of rhyolitic ignimbrites linked to large caldera-forming events at sources outside the field area, but locally sourced andesite and rhyolite lavas and domes are also encountered. Most of the rocks are allocated to the pre-0.35Ma Tahorakuri Formation. Crystallisation age spectra (and consequent best estimates of eruption age) have been obtained by U-Pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is an ignimbrite, which yields an eruption age estimate of 1.85±0.06Ma. This ignimbrite, plus comparable-aged units dated at the adjacent Rotokawa and Ohaaki geothermal fields, are interpreted to represent the oldest silicic volcanic deposits in the area, and onlap the basal andesite lava pile that is best developed at Rotokawa to the south. Other pyroclastic units and associated volcaniclastic sediments (with another intercalated andesite lava unit) return age estimates between 1.85±0.06 and 0.701±0.039Ma. Between ~0.7 and 0.35Ma, contemporaneous surface lithologies in the Ngatamariki area are dominated by sediments, with subordinate lava domes. Between 0.716±0.017 and 0.655±0.016Ma at least three shallow (to<2km depth) intrusions were emplaced under the northern part of the field: a diorite, microdiorite and a large tonalite body totalling >5km3. The intrusions generated a large alteration halo (~25km3 minimum) and intense silicification of the wall rocks. At 0.35 and 0.34Ma the area was buried by two ignimbrite packages of the Whakamaru Group, erupted from sources just west and well north, respectively, of the field. Ignimbrites of the Waiora Formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0.282±0.011 and 0.257±0.011Ma, coeval with more extensive volcanic activity in the Maroa dome complex west of the field. Sediments of the Huka Falls Formation and deposits of the 25.4±0.2ka Oruanui eruption then cap and seal the system. The new U-Pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the Ngatamariki Geothermal Field. The field, despite >2.8km of subsidence, does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of Quaternary age. Two chemically and temporally distinct hydrothermal events are located at the Ngatamariki field, with no evidence of continuity between the two

Extraction, Storage and Eruption of Multiple Isolated Magma Batches in the Paired Mamaku and Ohakuri Eruption, Taupo Volcanic Zone, New Zealand

ISSN:0022-3530ISSN:1460-241

A zircon U-Pb geochronology for the Rotokawa geothermal system, New Zealand, with implications for Taupō Volcanic Zone evolution

A U-Pb zircon geochronology for the Rotokawa geothermal system, central Taupō Volcanic Zone (TVZ), New Zealand, provides new constraints on the chronostratigraphy and volcanic and structural evolution of this area and the broader TVZ. The 3-km-thick volcanic sequence at Rotokawa is mainly composed of rhyolitic ignimbrites linked to large caldera-forming events from sources outside the field area, but locally sourced andesite and rhyolite lavas and intrusions are also present. Crystallisation age spectra (and consequent estimates of eruption age) have been obtained on zircons from hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is a Tahorakuri Formation ignimbrite (eruption age estimate of 1.87 ± 0.03 Ma) which, along with comparable-age units at other nearby TVZ geothermal systems (Ngatamariki, Ohaaki), is among the oldest silicic volcanic deposits known in the TVZ. These ignimbrites collectively onlap a basal andesite lava pile, up to 1.2 km thick at Rotokawa, that in turn rests on the Mesozoic basement greywacke. The base of the lava pile is more faulted than its top surface, implying that rifting and graben formation had started along the line of the modern TVZ arc prior to 1.84 Ma and is not a younger feature. Between ~1.8 Ma and 700 ka, there are no rocks represented at Rotokawa, with the next oldest lithology being a 720 ± 90 ka rhyolite lava. At 350 ka, the Rotokawa area was buried by regionally extensive ignimbrites of the Whakamaru Group, which have since subsided by ~700 m but not been greatly faulted. Ignimbrites and sediments of the Waiora Formation were then emplaced, coevally with widespread and volumetrically greater volcanism in the Maroa and Ngatamariki areas 13 km northwest and 8 km north of Rotokawa, respectively. Local rhyolites of the Oruahineawe Formation, dated at ~100 ka, were emplaced both as extrusive domes and shallow intrusions below the area. Sedimentary rocks of the Huka Falls Formation and deposits of the 14C-dated 25.4 ± 0.2 ka Oruanui eruption capped and sealed the system, which has since been disrupted by hydrothermal eruption events. The largest of these occurred at ~6.8 ka (14C date) broadly coincident with a resumption of eruptive activity at Taupō volcano, 20 km to the south-southwest. Notable aspects of the evolution of the Rotokawa area are the early onset of rifting and subsidence along the line of the modern arc, the lack of volcanic activity for N1 Myr from 1.84 Ma to 720 ka, the lack of faulting and only modest subsidence since 350 ka, and the contrasts in volcanic and subsidence histories with other, nearby geothermal systems