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Composite dykes, Porto Belo terrane, Zimbros Beach, Santa Catarina, Brazil



Roberto Weinberg and Fatima Bitencourt

Monash University and Universidade Federal do Rio Grande do Sul




This site records the intrusion of a composite dyke: mafic and felsic and their hybridization. Dyking efforts cause faulting of the host rock and these faults are exploited by the intrusive magma itself, causing blocky, angular contacts with the host rock. Photos from outcrops close to Praia Zimbros in Porto Belo, Santa Catarina, Brazil




stretched mafic
Hybrid dyke, with stretched mafic enclave with blue quartz and K-feldspar xenocrysts from surrounding rhyolite.
Sheared rhyolite dyke
Hybrid rhyolite dyke margin with strong lineation, plunging to SW and sinistral shear sense.
Hybrid rhyolite
Detail of rhyolite showing sheared blue quartz and K-feldspar.

composite dyke
Composite dyke, 4m wide with mafic enclaves in granitic matrix. Margin marked by fine-grained mafic dyke.
Mafic pillow inside granite with rounded and mantled K-feldspar.
composite dyke with xenolith
Composite dyke: mafic pillows, the central one has a xenolith of granite similar to the country rock.
dyke margin
Margin of main dyke has been exploited by a mafic dyke that crystallized into a fine-grained rock. Margins have angular indentations exploiting faults detailed in the following photographs.
faulted dyke margins
Dyke margin and apophysis separated by a granite septum, displaced by faults. These two photographs show how the faults in the host rock developed as a conjugate pair in response to the efforts related to dyking and were exploited by subsequent dykes that displaced or eroded the marginal blocks leading to angular displacements.
dyke margin
Margin of main dyke with a fine-grained mafic dyke. Margins have angular indentations. These have two origins: in some cases they exploit pre- existing fractures and in some cases these fault the narrow dykes. These fractures are interpreted to represent a conjugate fault pair developed in response to dyking. These faults are both exploited by late intruding dykes as well as displace earlier dykes.
faulted dyke margins
Example of the conjugate fault pair that in some cases displace both sides of the thickest dykes shown in some cases not. The fact that the faults do not cut across the thinner dyke below suggest faulting occurred in the time interval separating the two intrusions.
dyke margin
Conjugate fault pair affecting two dykes.