Uncommon Magmatic Structures, Borborema Province

K-rich granites of the Borborema Province in NE Brazil present a large number of uncommon magmatic structures which convey the dynamic history of the magma chamber. Some of these structures were of relatively simple interpretation (Weinberg et al., 2001). However, there are many other enigmatic features which are presented here.

These structures are more than curiosities. They tell us something about chamber dynamics. The purpose here is to illustrate the structures and trigger discussions, ideas and potential collaborations. If you want to help expand this site or connect your own related page to this one please let me know.

Weinberg, R.F., Sial, A.N., and Pessoa, R.R. 2001, Magma flow within the Tavares Pluton, NE Brazil: Compositional and thermal convection. Geol. Soc. Am. Bull. 113, 508-520 + cover image.

Magmatic Structures

Feldspar Aggregate Structures

     Random aggregate patches in homogeneous granite
     Aggregate layers: unidirectional protrusions
     Ellipsoidal structure

Feldspar Aggregates and Mafic Enclaves

     Aggregates at enclave margins
     Aggregate spheroid and mafic enclave dyke
     Other aggregate-mafic association

Granite Layering

Ellipsoids and Snails

Interstitial Melt Extraction

Feldspar Deformation: Magmatic and Solid-State

Dendritic Feldspar

Feldspar Aggregate Structures

Random aggregate patches

A) Boqueirao Pluton	B) Itaporanga Pluton

Aggregate Layers: Unidirectional Structures

A) K-feldspar megacryst aggregate forming a 20m long band (to right the photograph). with a flat straight bottom and a hilly top. The country rock is a megacrystic granite/granodiorite, Tavares Pluton.	B) More mafic granitic batch (at the bottom) associated with feldspar aggregate, Campina Grande pluton. See also Aggregates at enclave margins, in particular Fig. F

C) Unidirectional mounds on a K-feldspar aggregate layer, Campina Grande pluton.	D) Same outcrop as C) and same orientation, Campina Grande pluton.

C) Detail inside aggregate

Aggregates at enclave margins

A) Megacryst aggregate filter pressed where mafic pillows separated	B) Curved schlieren indicating magma flow in between enclaves

C) Megacryst aggregate around mafic pillow

D) Helicoid: mafic enclave and nest of megacrysts rotated by magma flow	E) Mafic enclaves surrounded by a nest of megacrysts, stretched by magma flow

F) Mafic enclaves surrounded by a nest of megacrysts, stretched by magma flow

Granite Layering

A) Complex schlieren pattern, Tavares pluton

B) Detail of A) showing shearing of schlieren

C) Layering, Itaporanga Pluton	D) Layering, Itaporanga Pluton	E) Detail of D)

F) Layering, Serra Branca pluton

G) Curved layering, Serra Branca pluton

H) Complex layering forming an elliptical shape, only half the ellipse shown , Serra Branca pluton

Magmatic Ellipsoids and Snails

Ellipsoids in Tavares pluton

A) Three ellipsoids, Tavares pluton

B) Ellipsoid, Tavares

Ellipsoids in tonalitic Tanzawa pluton, Japan, courtesy of Dr Tetsuo Kanamaru, Kobe University, and Dr Ryo Anma, Tsukuba University

Magmatic Snails

A) Snail structure, Tavares pluton

B) Snail structure, Tavares pluton

Magmatic Diapirs, Tavares pluton

A) Diapir delineated by schlieren	B) Enclave-rich diapir in granite
C) Diapir with poorly developed ladder structure along stem	D) Magma channel depicted by schlieren margin. Diapir stem?

Ladder Dykes

Weinberg et al. (2001) explained these structures as marking the stem of a rising plume or diapir where the source moved in relation to the exposed surface, so it marks the path of the source, like a hot spot. There are other less well-developed examples of ladder dykes in other sections of this website (e.g., Complex Dykes below). Ladder dykes and snails are thought to be formed by similar processes and Ellipsoids and Diapirs to represent the leading head of the rising magma batch on top of the stem.

A) Long ladder dyke, Tavares pluton

B) Detail of A

E) Begining of dyke in A), Tavares pluton	D) Detail of dyke in A), Tavares pluton
E) Poorly developed ladder dyke, Conceicao das Creoulas pluton

Complex Dykes

These are a group of dykes mostly from the Tavares pluton with complex internal structures. Some of which develop into ladder dykes (C and D), some are split along strike in one mafic part and a felsic part (E and F, also D), G) has irregular schlieren roughly parallel to the walls, and H) is a composite dyke formed by an early darker rock intruded by a later whiteish dyke, which cuts across the early dyke from right to left, upwards in the photo. At present most of these features remain unexplained.

A) Dyke breccia, Itaporanga pluton	B) Several interlinked complex dykes
C) Ladder dyke going into K-feldspar megacryst rich magma	D) Mafic and felsic dyke at the base grade upwards into ladder dyke

E) Mafic dyke turns to felsic halfway	F) Mafic and felsic complex layering in dyke
G) Schlieren in dyke with non-planar walls	H) Composite dyke (later felsic dyke) cutting across layered granite

Flow structures

Magmas in the Borborema plutons have a number of internal flow markers, such as the diapirs above, delineated by schlieren, other mafic schlieren marking flow between enclaves, and also as shown here in A) magmatic layering marks flow around a rigid block, and enclaves mark the flow of different internal magma batches (B and C). poor magma batches

A) Magmatic flow around an angular autoclave of granite, Tavares pluton

B) Internal contact between magma batches, Itapetim pluton

C) Deformed enclaves forming an arc at the margins of an internal flow (white arrows), same outcrop as B), Itapetim Pluton

Interstitial Melt Extraction

Segregation structures are thought to represent filter pressing of interstitial melt from a crystallizing magma with a solid interconnected network, effectively behaving like an isotropic migmatite. All structures are from Tavares pluton

A) Felsic melt flowed into an opening in rigid magma	B) Felsic interstitial melt diapir, extracted from crystallizing magma
C) Liquefaction structure in an isotropic granitic magma causing separation of mafic minerals from felsic melt and horizontal layering

Generally homogeneous granite becomes heterogeneous with mafic-rich bands in the vicinity of an irregular patch of leucocratic granite, suggesting it formed from local extraction of interstitial evolved melt, as magma crystallized. In its entirety, these extraction structures represent a filter pressing step in magma fractionation.

D) Irregular 3D extraction structure	E) What is going on here?

F) Residual material (mafic) from extraction of interstitial melt frozen in pockets (leucosome). Extraction channelways? Same outcrop as B,C and D.

Complex extraction features?

G)	H) Detail of G)

Magmatic and Subsolidus Feldspar Deformation

Some beautiful megacryst deformation in Itaporanga and Pedra Mijada plutons. In Itaporanga many megacrysts are bent as in (A) suggesting squeezing of interstitial magmas and megacryst deformation

A) Squeezed and bent cumulate K-feldspar. Is this magmatic?	B) Tail in K-feldpar megacryst (5 cm long), accompanied by other grains with no tails.
C) Solid-state zoned K-feldspar from Pedra Mijada Pluton for comparison