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Campina Grande Petrography

Roberto Weinberg, Monash University, Australia

Ignez Guimaraes, Universidade Federal de Pernambuco

Neysi Almeida, Universidade Federal de Pernambuco

 

 

 

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL: users.monash.edu.au/~weinberg.

 

I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.

 

DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.

 

Introduction

In the Campina Grande pluton there is evidence for the mingling and mixing of pegmatitic granites and diorites. This is unusual and because of the combination of low viscosity and low solidus temperature of the pegmatitic melts, combined with extreme fractionation, the mingling of these two melts has many distinctive features when compared to the mingling of granite and diorite magmas.

Morgan and London (1987, Am. Mineral. 72, p. 1097-1121) describe the metasomatic alteration of a pre-existing amphibolite upon intrusion and crystallization of an evolved pegmatite rich in rare elements. They determine three stages of alteration. The first stage defined is characterized by B-alteration, and is associated with tourmaline, as well as tit, calc etc. The second stage was termed the K-Rb-Cs-F alteration and lead to the growth of bt. The third stage was propylitization with the alteration of hbl+pl to ep+chl+tit+cal+ clay. We found that during the mingling hystory, that a modified version of the same pattern can be followed.

 

Major Summarizing Features

Contact felsic-mafic:
Dendritic at a range of scales. Growth of large hbl crystals (FD01, FD16), tit (FD16) and opaques (FD01). Py, minor cpy, enriched at mingled contacts. Best developed in FD01, where a large op grows at contact and small grains are more common close to the contact.

Diorite close to granite contacts develop two types of characteristic textures: poikilitic or orbicular honeycomb textures.

 

Poikilitic texture:
crystals of plagioclase (e.g. FD4a) and bt (eg FD4a, b). In some cases the poikilocrystals are interstitial and zoned (FD3b), in one case hbl is also poikilitic (FD3b). Biotitization of the diorite is a typical feature of contact diorite+pegmatite.

 

Orbicular honeycomb texture :
samples FD3b, 6.8A, 09 and 13. Zoned orbicules with hbl surrounding cores of aug+op or aug+qz (symplectite). These orbicules have few bt. Typically these rocks have a very characteristic “orbicular and honeycomb” texture, where the orbicules are embedded in a fine-grained mesh of predominantly bt+pl and minor hbl. In some cases you have bt around hbl (best around FD6.8A). Typically hbl has tiny qz worm inclusions (best developed in FD6.8A). Sometime rather than orbicules there are amph. aggregates (FD6.8A).

 

Hydrothermal alteration:
Paths of alteration to greenschist facies minerals (act+chl+ep (+ab?)) FD201b (best example), blebs of aug within hbl, ep alteration at contacts hbl-pl (use FD5.1 to define the nature of these pathways) Paths of pl alteration to dust and sericite, in one case alteration follows a zone around a crack: FD201b, FD3b.

Narrow zones of Kf recrystallization (FD16, best in FD5.1A), which I suspect is alteration, possibly to ab, and possibly tml or act?

 

Tourmaline in diorite:
Tourmaline grains are found within hornblende (as inclusions; sample FD202) as well as within pegmatite (FD06.9), suggesting a B alteration of the diorite. The timing of alteration is unclear but the fact that tml is found within well formed diorite hbl suggests early formation.

 

Biotite surrounding plagioclase parallel to faces. FD16

 

Interpretation: the poikilitic, orbicular textures are a signature of instability during mixing

 

Question: is the hydrothermal greenschist facies alteration, and patches of act in hbl a result of alteration from fluids released by low-temperature crystallization of pegmatites (400-500 C)?

 

If this is so, mingling has a super-solidus effect on diorite giving rise to the textures and growth of bt through possible addition of potassium, and a sub-solidus effect with partial alteration to greenschist facies assemblages. The latter might be impossible to solve, but all of it can be indicated by geochemistry.

 

 

FD12.3: Bt-hbl-gr leucogranite

 

Texture:porphyritic aggregate/cumulate texture deformed syn- magmatically (broken phenocrysts with cracks intruded by qz+bt) and subsolidus (recrystallized qz)

 

Plagioclase:
subedric laths with irregular margins in one grain a beautiful single lamella altered to a single ms crystal and another with a wider band of bt occurs also as altered (cc+ep+ser and dust) inclusions in Kf some large crystals are beautifully fractured with intervening qz+bt lammelae extinction angle 5

 

K-feldspar:
phenocrysts, myrmekite, tartan twinning cracked, cracks filled by qz + ep inclusions of altered pl

 

Quartz:
Matrix finely recrystallized to perfect new grains

 

Biotite:
In plag marginsand in small aggregates with hbl

 

Hornblende
green pleochroism, small irregular grains

 

Accessories: titanite, apatite, garnet (isotropic hexagons much larger than any apatites, occur within a mafic aggregate)

Alteration: chlorite, cc, epidote

 

Photographs to do: sphene in crack within phenocryst, altered single lamella, cracked phenoc and indentation

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FD13: Orbicular diorite contact with porphyritic granite

A) Orbicular diorite contact, 2.5 cm view Orbicular diorite

B) Two zoned orbicules, 5 mm across Orbicular diorite

C) Honeycomb texture, 1.5 mm across honeycomb texture

D) Honeycomb texture, 1.5 mm across honeycomb texture

 

Texture. diorite: orbicules in honey-comb mesh, different from other honey-comb textures in C.G. this sample has hbl+bt+pl in the honeycomb mesh, while others have bt+pl and hbl grows separately and tend to form larger isolated crystals (see FD09 and 6.8A).

 

Diorite: 1-5 mm elliptical nodules of pure of amphibole with 10% bt laths but no plag. Nodules embedded in a fine rock of hbl+pl+bt. Nodules are zoned with pleochr. hbl surrounding aug+tiny opaques in nodule cores (weak green pleochr.)

 

Irregular contact with granite. There is bt aggregate within the granite linked to the diorite through a very narrow isthmus, and this aggregate has no amphiboles, unlike all mafic aggregates in diorite suggesting amphiboles became unstable.

 

Plagioclase:
poikilitic interstitial (bt+hbl inclusions)

 

Biotite:
Forms an aggregate within granite but physically linked to diorite and associated with lots of sphene, zircon, and large apatite

 

Hornblende:
green pleochroism

 

Augite:
perhaps a few remnants

 

Augite:
weak pleochr.

 

Accessories: titanite, zircon staining bt

 

Pegmatitic granite

 

Kf phenocrysts: irregular with plag inclusions and apatite

 

Quartz: recrystallized matrix

 

Biotite

 

Accessories: sphene apatite, py, cpy (opaques)

 

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FD09: Diorite mingled with granite

 

Texture:Orbicules in meshed matrix diorite mingled with sheared granite

 

Amphibole orbicules are zoned with hbl surrounding aug cores which have small qz(?) worms (see also FD3b and FD13). Orbicules are embedded in a fine matrix that is a mesh of predominantly bt and pl, and less hbl plus accessories
A) Orbicular diorite contact, 2.5 cm view Orbicular diorite

B) Two zoned orbicules, 7 mm across Orbicular diorite

C) Zoned orbicule, 3.5 mm across Orbicular diorite

D) Honeycomb text, 1.5 mm across Orbicular diorite

 

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FD201B: Hbl-bt-aug diorite hydrotermally altered

 

Texture:

 

Plagioclase:
well-developed twin lamellae, not zoned, and poorly twinned zoned, altered to dusty plag and sericite along pathways and irregularly showing fluid paths. Extinction angle 23-24. Deformation: recrystallized grains, bent lamellae and fractured grains .

 

Amphiboles:
strongly pleochr. hbl, weakly pleoch. aug. Inside hbl grains there are patches of preserved aug and fine grained ep (one case where a large ep grain grows inside a hbl crystal and is surrounded by an augrim). In many cases act seems to grade to hbl.

 

Fluid pathways:
can be tracked trough the sample causing the break down of hbl+pl to ep (along mutual contacts between hbl+pl) and hbl to act. Sometimes there is evidence for a breakdown of hbl into ep+act in fine intergrowth, or hbl to ep+qz (?). Bt is locally altered to chl (anomalous blue pleoch.)

 

Epidote:
lots (not so obviously primary as in some other samples of the C. Grande)

 

Accessories:
tit, ap

 

Alteration:
ep, sericite, chl

 

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FD03B: Pan-poikilitic diorite

 

Texture:
fully poikilitic, large zoned interstitial pl including hbl + bt, large hbl

 

 

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FD04A: Pan-poikilitic diorite mingled with pegmatite

 

Texture:
Poikilitic bt phenocrysts (>1cm long) with inclusions of aug (weak green) surrounded by grains of hbl (green pleochr)

 

Granite

 

Quartz:
in granite 1 cm across (normally 0.5 cm or less) irregular shape

 

Microcline:
perthite, up to 0.5cm across, flame perthite, anhedral, dirty aspect. Plagioclase zoned poikilitic anhedral interstitial up to 8mm wide continuous across other grain masses within granite (see FD3b).

 

Biotite:
few small grains Irregular contacts with diorite

 

Diorite

 

Few pl grains in mafic part.

Hornblende:
forms phenocrystals as well

 

Augite grade to hbl with stronger pleochroism

 

Biotite:
poikilitic phenocryst (3-8 mm long) slightly deformed. There is a tendency for amphibole inclusions in bt being transparent or weakly green pleochroic compared to those outside. Lots of epidote inclusion

 

Epidote:
inclusions in bt

 

Rapid cooling and addition of water and K gives rise to pan-poikil. texture and growth of bt.

Difference to other rocks is that this one is intensely retrogressed with few hbl left.

 

 

FD04B: Pan-poikilitic diorite

 

Texture:
Poikilitic bt phenocrysts (>1cm long) with inclusions of aug (weak green) surrounded by grains of hbl (green pleochr)

Hbl forms phenocrystals as well

Plagioclase zoned poikilitic anhedral interstitial up to 5mm wide continuous across other grain masses (like FD3b). Hydrothermal alteration gives rise to dirty plagioclase randomly spread over the thin section (not defining pathways).

Rapid cooling and addition of water and K gives rise to pan-poikil. texture and growth of bt.

 

A) Poikilitic biotite

B) Poikilitic biotite

C) Poikilitic intestitial plagioclase D) Poikilitic intestitial plagioclase

 

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FD5.1A: Granodiorite mingled pegmatite

 

Texture:

 

Granodiorite: bt-rich and no amph. nodules bt+hbl+pl+ep+all

 

When mafic protrudes into felsic you have what looks like a recrystallized band within large Kfeld where there are op, bt, myrmekite and titanite (seed also FD6.8) and fibrous tremolite (could it be tml?). It is possible that these “recrystallized” paths are really true mixed material and these can be followed for over 2cm. These recrystallized paths follow the boundaries of Kf.

 

Accessories: lots of ap and zr, all, ep

tit: as alteration product, xenomorphic ep+all: forming corona textures, euhedral ep. 0.5 mm across, with a core of all. 0.2 mm. Four grains like this, all associated with bt.
A) Ep+all corona B) Ep+all corona

 

Chlorite alteration (one grain)

Qz normally well formed coarse and underformed suggesting that the fine bands in Kf and their contacts are not a result of deformation but a result of fluid paths.

 

We need to establish what is in these paths: they are either fluid paths released from the mafic protrusion or whether they are regional fluid paths.

 

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FD6.8A: Diorite mingled pegmatite

 

Texture:
honeycomb (this is the best mesh) and aggregates. amphiboles form aggregates of the same nature of orbicules but do not have the orbicule shape. Out of a protrusion of mafic material there is micrometer thin band of amphiboles within pegmatite. Mafic protrusion englobe single pl crystals from pegmatite. Tit and op (cpy+py) develop at the tip of mafic protrusions.

 

Amph. form phenocrysts up to 4 mm long, generally about 1 mm long and are also in matrix. Some aug have very well developed qz worm inclusions and they are surrounded by bt grains (in one case 100% of the amph).

 

Allanite (single grain at contact Pegm-dior)

 

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FD6.8B: Diorite mingled pegmatite

 

Texture:orbicular honeycomb (not zoned orbicules)

Hbl phenocr with qz worms, hbl aggregates cored by aug?, small grains aggregate

 

There are two sorts of qz inclusions in hbl-aug. One is a wormy narrow qz related to hbl loosing pleochr, another which are larger rounded qz blobe which could be just plain inclusions in hbl that do not look altered to act.

 

The aggregates like in other thin section are bt poor

 

Like in sample FD4B there are fluid pathways evidenced by alteration (but not very clear)

 

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FD202: Deformed tml diorite

 

Texture:
phenocrysts of plag and Kf (few perthite exsolution). Deformation: crystal preferred orientation and gneissosity. Fracture band cuts across thin section filled with very fine brown pleochroic material (possibly bt).

 

Hbl: yellow to green pleochroism, seriate size distribution, includes ep. in core and ap, some grains look like hbl embayed by qz. Hbl has small wormy inclusions of qz, emabayed pl, and tml inclusions. Hbl phenocryst with a core that looks to be aug (weak green pleochr.)

 

Tml: inclusions in hbl. Zoned pleochroism from brown to yellow, anomalour interference colours depicting zoning from brown to blueish colours. Inclusions are either elongated but not needles or rounded

 

Tml+pl inclusion in hbl

 

Aug: associated with hbl alteration to epidote

Pl: zoned, twinning, bent lamellae, <30 degrees extinction angle (andesine) up to 3mm

Kf: up to 3mm equidimensional include mafic aggregate of bt+op+sph+ap and pl

Epidote: well formed tending to euhedric but some anhedric, high relief, transparent grains associated with hbl alteration Qz: intergrown with hbl, similar to that in other mingled mafic rocks.(? meaning?)

Tit: large poorly developed grains, interstitial anhedric and stained surface

Matrix <0.5 mm

Ap: Lots of ap grains, long needles and well formed grains.

 

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FD08: Leucogranite

 

Texture:
Coarse, dirty, hydrothermally altered leucogranite

Fibrous tourmaline: light brown to light green pleochroism growing around a grain of qz Biotite: light brown to dark brown forms shlieren associated with opaques, specially around individual isolated bt grains + ap+ep+zr+tit. Bt has a pointy, spidery shape

 

Tourmaline: tml, long gerains pleochroism brown to light brown, right angle extinction, grows close to bt, one grain 0.1 mm long, 0.015 mm wide Opaque trail, interstitial, 6 mm long 1 mm wide

 

Kfeldspar: flame perthite, myrmekite, 3mm long, poorly developed Karlsbad twins.

 

Plagioclase: dusty, equant upto 2mm, some qz intergrowth, and pl as inclusions with corroded margins in Kf 0.5 mm in size

 

Quartz: new grains/ondulose extinction upto 4mm across

 

Epidote: in bt aggregate, sometimes euhedral

 

Zircon: many grains in bt aggregate, causing stains in bt. Lots of zr everywhere and other unrecognized alteration minerals. Finely recrystallized material within Kf (0.1 mm wide band) which looks like fluid paths (no minerals could be determined)

 

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FD01: Diorite mingled with granodiorite

 

Texture:
Mingled unstable texture with incipient orbicular texture.

 

Diorite: hbl, pl, bt diorite, fine grained, 0.1-0.2mm long with hbl phenocrysts, up to 1.5 cm close to granodiorite pods, as well as orbicules up to 1.5 mm wide, with fine opaque grains and aug in the core. There is a phenocrystal with opaque grains, and other hbl aggregates, orbicular like but without aug and the rounded shape. This is a case of incipient orbicular text. and the orbicules, aggregates and phenocrysts aqre all close to the granodiorite pods indicating that they are a result of mingling.

 

Plagioclase: patches where they are saussuritized and dusty, other areas plagioclase are pristine.

 

Opaques: tend to be heterog. distributed, small grains in dior. are richer close to the granodiorite pods. Large opaque, poikilitic phenocrystal at contact (2 mm).

 

Granodiorite pods:
these are coarse grained hbl-poor, bt-rich, and titanite altered pods, with margins rich in coarse hbl and op.

 

Plagioclase: tend to form continuous grains without clear boundaries. Saussuritized and with large calcite grains plus op and seric as alteration.

 

? high birefringence colour, straight extinction, fibrous alteration mineral(?) related to pl.

 

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FD6.9: Mingled diorite and tml pegmatite

 

Texture:
Deformed and locally mylonitic pegmatite

Bt in pegmatite: single crystal >1cm across, beautiful internal deformation, within pegmatite. Includes long needles of radial tml (right-angle ext). My reading of the ts, is that early formed bt grains were subsequently altered by the pegmatitic fluids to tit+tml along bt grain margins

 

A) Tml needles in bt B) Tml needles in bt

 

Qz: fully recrystallized coarse grains

Pl: deformed

Kf: deformed, mylonitic very fine grained. Coarse, with myrmekite on margins

 

No alteration products related to mylonitization of feldspars such as sericite, cc or ep, suggesting amphibolite facies deformation

Tml: occurs as inclusion needles in bt, but also as fine amorphous crystal swarms with green pleochroism which I thought first to to be hbl, but has right-angle extinction in long sections

Mafic rock is composed of hbl+bt+pl+Kf+ accessories

Biotite in diorite masses of zircon inclusions (>50 grains in a small patch; high relief, prismatic, high interference colours) as well as ep+tit.

 

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FD200: Mylonitic mingled granite-diorite

 

Texture:
Porphyritic band of felsic ultramylonite within a diorite partly ultramylonitic and partly preserved (beautiful)

 

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FD16: Mingled granite-diorite

 

Texture:
preferred orientation due to deformation. Contact mafic-felsic characterized by largest grains of hbl, large tit

 

Diorite:
Hbl: dark green pleochroism, local crystal faces, well formed twins surroudnde by fine groundmass of bt. Preserved aug in a few crystals. Hbl unstable: pleochroism becomes pale suggesting aug. Also intergrowth/ inclusions of quartz

 

Aug: in hbl

Pl: large crystal (ovoids), recrystallized, bent lammellae. Surrounded by bt parallel to faces

Kf: microcline perthitic

Accessories: tit, ap, op, zr

 

Contact:
large hbl grains
tit: at contact, large, twinned grains
Kf: recrystallized, complex internal deformation

 

Granite:
Kf: tartan twinning, deformed internally recrystallized. Fine bands of recrystallization (like many other samples).

Qz: fully recrystallized to very small grains

Bt: very few

 

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FD11.1: mylonite in pegmatite (ts broke)

 

Texture:
mylonite developed in pegmatite. High-T deformation of pegmatite comparable to FD6.9. Throughout the entire section there are parallel bands of dusty alteration overprintint the feldspars (never seen anything like it). Mylonite band separates a region characterized by phenocrysts of feldspar (Kf no twinning no perthite) swimming in recrystallized qz, from a zone of much coarser microcline (1.5 cm) fully perthitic and with well-developed tartan twinning.

 

Kf: on ome side of mylonite phenocrysts fractured and broken embedded in a recrystallized qz matrix. Beautiful high-T bending of Kf lammellae (broad kink bands), and finer grained recrystallized Kf tails. No twinning, no perthite (one little patch found), cut by parallel dusty lines, ondulose extinction and recrystallized. On the other side the microcline is twinned, perthitic and coarser 1.5 cm long (a different phase altogether).

Pl: unaltered, cut by parallel dusty lines of alteration

Qz: polygonal recrystallized grains

Bt+ep: low modal percentage

Ep filled cracks across Kf Hbl: green inclusion in Kf (is it tml?)