Arizona Geological Society
2026 Speaker Series
Tuesday, 1 December 2026 | 5:30 - 8:00 PM
Location: Hexagon Mining Division Office
40 East Congress Street, Suite 150, Tucson, Arizona 85701
Parking: On the street or parking garage (Old Pueblo Parking)
Social Hour - Sandwiches from Beyond Bread (5:30-6:20 PM MST), Presentation (6:25 PM MST)
AGS and SEG Publications will be available
for sale at the meeting.
Will need cash or check (no credit cards)
For those planning to attend the event, please register by 6:00 PM on Sunday, November 29, 2026. Please register early so we have an accurate number of attendees to order the right amount of food.
The Arizona Geological Society thanks Ivanhoe Electric for sponsoring our students' dinners at our monthly meetings and bi-annual field trips.
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The Arizona Geological Society thanks Hexagon
for generously providing the venue and drinks
Geochronology and Geochemistry of Mesoproterozoic (1.4x Ga) Granulite Facies Metasediments and Associated Rocks of the Sierra Estrella and South Mountain near Phoenix, Az: A Rare Window into the Lower Crust of Southern Arizona, Subduction Processes Associated with the 1.4x Ga Mazatzal/Picuris Orogeny, and the Origin of 1.4x Ga Grantoid Suite of Southern Arizona
Arend Meijer, University of Arizona , Tucson, Arizona
Abstract: The Sierra Estrella and South Mountain are two desert ranges directly south of Phoenix, Arizona, USA both located within the 1.6-1.7 Ga Pinal terrain, a Paleoproterozoic subduction complex. The Pinal terrain is composed largely of foliated fine to medium-grained greenschist facies metasediments although it also contains local accumulations of metavolcanic rocks, some ultramafic rocks, and scattered forearc granitoid plutons thought to have resulted from a spreading ridge subduction event at 1.65 +/- 0.020 Ga. The Sierra Estrella is made up of a heterogeneous collection of metasediments, metagranitoids, gneisses, with lesser mafic rocks, granitic pegmatites, and minor Tertiary rock units. Zircon dating indicates that most of these rock units are essentially more highly metamorphosed (amphibolite facies) and tectonized equivalents of rocks exposed elsewhere in the Pinal terrain. However, in the northern Sierra Estrella, an exposed metasedimentary assemblage that was metamorphosed to granulite facies has yielded 1.4x Ga zircon ages. Nearby exposures of mafic garnet granulite of 1.4x Ga vintage are part of this assemblage. Western South Mountain is made up of amphibolite to granulite facies metasediments and gneisses that also appear to be largely of 1.4x Ga vintage. Major and trace element analyses of mafic units associated with the 1.4x Ga granulite facies metasediments in the northernmost Sierra Estrella and 1.4x Ga amphibolite facies rocks in western South Mountain indicate these units have element concentrations very similar to those observed in modern oceanic island arc lavas. A logical explanation for these observations is that a 1.4x-1.5x Ga oceanic island arc and associated sediments were subducted beneath the Pinal subduction complex at approximately 1.38-1.41 Ga resulting in a duplexed subduction complex. The subduction of this oceanic arc apparently produced a limited volume of 1.38-1.40 Ga granitoid magma that intruded the overlying duplexed subduction complex. However, the larger and somewhat older (1.43-1.45 Ga) plutons of southern Arizona (e.g., Oracle granite, Ruin granite, Continental granite, etc.) were intruded into the Pinal subduction complex prior to the arc subduction event. Isotopic data indicate the source materials for these plutons included variable proportions of Pinal subduction complex materials and mantle-derived magmas. Partial melting of the Pinal subduction complex in this setting would have required the existence of an enhanced thermal regime within the complex. It is here suggested that this regime resulted from the subduction of an active (back-arc?) spreading ridge. It is further speculated that the subduction of an active spreading ridge followed by subduction of an oceanic arc may have been causal factors in the 1.38-1.45 Ga Picuris/Mazatzal orogeny.
Bio: Arend Meijer started his career working on the evolution of the Mariana arc system with Dan Karig. Dan worked on the tectonic aspects and Arend worked on the petrologic/geochemical aspects. This work established a new paradigm for understanding the evolution of oceanic arc systems that has stood the test of time. It also resolved the question of whether or not oceanic sediments (i.e., continental crust) are physically recycled into Mariana arc magmas upon subduction. They are not. This and later studies put an end to an idea championed by R. L. Armstrong that continents had formed early in earth history and had simply been recycled in subduction zones since then. During this time, Arend dredged basalts from the Mariana trough on Scripps oceanographic ships and was a shipboard scientist aboard the drilling ship Glomar Challenger when it drilled an east-west transect (D.S.D.P. Leg 60) across the Mariana arc system from the Mariana Trench to the western portion of the Mariana Trough (i.e., backarc basin). The results obtained on this cruise were entirely consistent with Dan’s model for the evolution of island arc systems. Another interesting result from this cruise was the discovery of boninites as an integral part of the juvenile Mariana arc system. While on the faculty at the University of Arizona, Arend also travelled to each of the islands in the Mariana arc for research and to collect samples for further petrologic/geochemical studies generally accompanied by a few graduate students. The second decade of Arend’s career was spent at Los Alamos National laboratory where he worked as a project leader on various classified and unclassified projects. His professional interests during this time included hydrochemistry. In the following two decades, he started a consulting company in chemical geology and hydrochemistry and had contracts with various oil companies, mining companies, chemical companies, and governmental agencies. Since retiring in Tucson, he has carried out research on the Proterozoic of southern Arizona including projects in the Yavapai, Mazatzal, and Pinal terrains, and has published several papers on these research efforts in professional journals. A manuscript on Sierra Estrella/South Mountain studies was recently submitted for publication and a manuscript on the Yavapai terrain is currently in preparation.
Hexagon Mining Division Office - 40 East Congress Street,
Suite 150, Tucson, Arizona 85701
