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GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 16 Continental Arc Magmatism March 2, 2016 CONTINENTAL ARCS VS ISLAND ARCS AFFECTS OF THICK DIFFERENTIATED CONTINENTAL CRUST Thick sialic crust contrasts greatly with mantle-derived partial melts may produce more pronounced effects of contamination Low density of crust may retard ascent causing stagnation of magmas and more potential for differentiation Low melting point of crust allows for partial melting and crustallyderived melts Subcontinental lithosphere may be more compositionally diverse that suboceanic lithosphere, especially if crust is old WHY ISLAND ARCS IN THE WESTERN PACIFIC AND CONTINENTAL ARC IN THE EASTERN PACIFIC? CONTINENTAL ARCS FORM WHERE THE SUBDUCTING OCEAN CRUST IS YOUNG Young Oceanic Crust is: Warmer – Dehydrates and melts more readily Lower Density - Bouyant More Ductile – Flexible, Deformable TYPES OF CONTINENTAL ARCS Destructive more common where Continental crust is older e.g. Andean Margin Constructive more common where Continental crust is younger e.g. Pacific NW ANDEAN CONTINENTAL ARC Gaps in volcanic activity • shallow subduction • overthickened slab ANDEAN VOLCANIC COMPOSITIONS DISTRIBUTION OF ROCK TYPES Melting of Lower Crust generates Felsic Magmas Lower Crust traps Mafic Magmas ANDEAN VOLCANIC COMPOSITIONS MAJOR ELEMENTS Northern Volcanic Zone • more andesitic to felsic • K-rich comps to east Alkaline Rocks Central Volcanic Zone • more andesitic to felsic • basalts rare • more staging beneath Precambrian crust Southern Volcanic Zone • broad range of comps • K-rich comps to east • shallower subduction angle • Young continental crust especially to south • Most similar to island arc magmatism Island Arcs ANDEAN VOLCANIC COMPOSITIONS TRACE ELEMENTS CVZ – Assimilation of Precambrian crust and SCLM SVZ - Shallower subduction angle melting of Gt-free mantle ANDEAN VOLCANIC COMPOSITIONS TRACE ELEMENTS CVZ – Assimilation of Precambrian crust and SCLM Nb-Ta depletion similar to island arc dehydration of subducted slab ANDEAN VOLCANIC COMPOSITIONS ISOTOPIC COMPOSITIONS CONSTRUCTIVE CONTINENTAL ARC PACIFIC NW Juan de Fuca Plate – Young, hot, bouyant; dehydrates quickly upon subduction Columbia Embayment area of young crust and arc construction by rollback or trench jumping CASCADE MAGMA TYPES OVER TIME Mafic Volcanism Bimodal Volcanism Mafic Underplating/Crustal Melting CalcAlkaline Volcanism Crustal Assimilation Crustal Extension = Back-arc? = Northward Migration of Basin and Range? CONTRASTING THE ANDEAN AND CASCADE ARCS Andean Arc Strong compressional tectonics promotes staging and contamination of mantle melts in thick, old crust Intermediate volcanics dominate Cascade Arc Roll-back extension promotes dominance of mafic volcanism. Silicic magmatism related to underplating/melting of the lower crust. GENERAL MODEL FOR CONTINENTAL ARC MAGMATISM MASH – Melting - Assimilation – Storage - Homogenization