Abstract

In the Winnipesaukee area of central New Hampshire two groups of igneous rocks are strikingly different. The New Hampshire magma series is probably late Devonian, and the White Mountain magma series is probably Mississippian. The contrasting characteristics of these two magma series are tabulated.

The New Hampshire magma series here consists of large partially concordant intrusions of Meredith porphyritic granite and Winnipesaukee quartz diorite and small intrusions of hornblende gabbro and Concord granite. Conclusive evidence of the origin of this series is lacking, but some evidence indicates that it has been derived by melting of sial.

The White Mountain magma series includes a great variety of extrusive and intrusive rocks in the Ossipee Mountains, the Belknap Mountains, and Red Hill. Cauldron subsidence is an important method of intrusion. The order of intrusion, from oldest to youngest, is generally gabbro, diorite, monzodiorite, syenite, quartz syenite, granite. Calculations indicate the nature of the materials which could be subtracted or added, to cause the changes shown by this sequence. These calculations suggest that neither subtraction, in the form of crystal fractionation, nor addition, in the form of assimilation or pure melting, is the sole explanation, but that both played a part. Crystal fractionation probably dominated up to the stage of syenite, and assimilation probably assumed increased importance in the change from syenite to granite.

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