The geological mapping of ancient stratovolcanoes reveals a portrait of their stratigraphy, structural architecture and chemical cyclicity formed in response to magmatism over periods typically spanning 100 to 1000 kyr. In back-arc settings, detailed stratigraphic studies are especially pertinent because these volcanic fields are characterised by long-lived, chemically diverse volcanism that is sensitive to changes in tectonic conditions (e.g. back-arc extension) associated with the down going slab. The Alexandra Volcanic Group (AVG) of western North Island, New Zealand is a prime example of a back-arc, basaltic volcanic field that preserves a record of chemically complex, arc to intraplate volcanism from the Late Pliocene to Early Pleistocene (2.7–1.6 Ma). The aim of this research is to reconstruct the volcanic history of Mt Pirongia, and its underlying arc to intraplate magmatic system in the context of the broader AVG. Detailed mapping on Pirongia has yielded the first 1:30,000 geological map of the volcano. Unit correlations are based on field and geomorphic observations, radiometric dates (with two new ⁴⁰Ar/³⁹Ar ages) and detailed petrography. The Pirongia Volcanic Formation is now subdivided into six new stratigraphic members and 36 new lithostratigraphic units, which collectively form the largest basaltic volcano (~30 km³) in North Island. The edifice has formed from at least 18 vents that define an ankaramite shield structure surmounted by three steep sided cones. Major cone-building stages are separated by sector collapse events. The largest collapse deposit is the 3.3 km³ Oparau breccia debris avalanche, which was channelised into its namesake graben and extends 20 km southwest of Pirongia. Collapse was likely triggered by normal fault reactivation and followed by resurgent volcanism at Pirongia Summit (1.6 Ma). A comprehensive set of 200 rock samples from Pirongia, Karioi, Te Kawa and Kakepuku were analysed for major oxide, trace element and (selected) Sr-Nd isotope compositions. The dataset shows that arc-type (IAB) basalt is volumetrically dominant in the stratovolcanoes, which are interspersed with monogenetic intraplate (OIB) basalts of the Okete Volcanics. Minor intercalation of IAB-OIB occurs at Pirongia and Kakepuku, whereas OIB lavas comprise almost half of Karioi. Elemental and isotopic data indicate that all arc lavas have compositions falling on a spectrum between IAB and OIB endmembers. Cross-arc geochemical variation indicates a northwestward enrichment of Nd-isotope ratios from Te Kawa to Karioi that corresponds to an increase in the OIB signature with increasing slab depth. Stratigraphically, Karioi is defined by sharp alternations of OIB-IAB volcanism, while Pirongia displays progressive and cyclical enrichment from IAB to transitional compositions. At Pirongia, sustained injection of OIBs into the IAB magma reservoirs has led to cyclical contamination and more OIB-like compositions through time. The juxtaposition of IAB and OIB is explained in terms of slab rollback, which induces vertical and lateral upwelling of asthenosphere, via slab tear and along the slab edge, into the metasomatized mantle wedge. Initial decompression melting forms OIB, which triggers secondary conductive melting of the wedge to form IAB magmas. This study demonstrates the importance of robust stratigraphy for unravelling complex chemical cyclicity in stratovolcanic systems.