G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 16 Continental Arc Magmatism March 2, 2016

C ONTINENTAL A RCS VS I SLAND A RCS A FFECTS OF T HICK D IFFERENTIATED C ONTINENTAL C RUST  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 crustally- derived melts  Subcontinental lithosphere may be more compositionally diverse that suboceanic lithosphere, especially if crust is old

W HY I SLAND A RCS IN THE W ESTERN P ACIFIC AND C ONTINENTAL A RC IN THE E ASTERN P ACIFIC ?

C ONTINENTAL A RCS F ORM W HERE THE S UBDUCTING O CEAN C RUST IS Y OUNG Young Oceanic Crust is: Warmer – Dehydrates and melts more readily Lower Density - Bouyant More Ductile – Flexible, Deformable

T YPES OF C ONTINENTAL A RCS Destructive more common where Continental crust is older e.g. Andean Margin Constructive more common where Continental crust is younger e.g. Pacific NW

A NDEAN C ONTINENTAL A RC Gaps in volcanic activity shallow subduction overthickened slab

A NDEAN V OLCANIC COMPOSITIONS D ISTRIBUTION OF R OCK T YPES Lower Crust traps Mafic Magmas Melting of Lower Crust generates Felsic Magmas

Island Arcs Alkaline Rocks Northern Volcanic Zone more andesitic to felsic K-rich comps to east 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 A NDEAN V OLCANIC COMPOSITIONS M AJOR E LEMENTS

A NDEAN V OLCANIC COMPOSITIONS T RACE E LEMENTS SVZ - Shallower subduction angle  melting of Gt-free mantle CVZ – Assimilation of Precambrian crust and SCLM

A NDEAN V OLCANIC COMPOSITIONS T RACE E LEMENTS Nb-Ta depletion similar to island arc dehydration of subducted slab CVZ – Assimilation of Precambrian crust and SCLM

A NDEAN V OLCANIC COMPOSITIONS I SOTOPIC C OMPOSITIONS Crustal Contamination

C ONSTRUCTIVE C ONTINENTAL A RC P ACIFIC NW Columbia Embayment - area of young crust and arc construction by rollback or trench jumping Juan de Fuca Plate – Young, hot, bouyant; dehydrates quickly upon subduction

C ASCADE M AGMA T YPES OVER T IME Bimodal Volcanism Mafic Underplating/Crustal Melting Mafic Volcanism Crustal Extension = Back-arc? = Northward Migration of Basin and Range? Calc- Alkaline Volcanism Crustal Assimilation

C ONTRASTING THE A NDEAN AND C ASCADE A RCS 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.

G ENERAL M ODEL FOR C ONTINENTAL A RC M AGMATISM MASH – Melting - Assimilation – Storage - Homogenization